nixos/test-driver: drop logging from Machine.send_monitor_command

Several machine operations, like `send_chars` and `send_key`, are
implemented by calling `send_monitor_command`, possibly multiple times.
This generates a huge amount of unnecessary noise in the log, because
`send_monitor_command` is a low-level operation and an implementation
detail.

Here's an excerpt from a highlighted log before and afte the change.

Before:

    subtest: Can generate a PGP key
    machine: sending keys 'gpg --gen-key\n'
    machine: sending monitor command: sendkey g
    machine: waiting for monitor prompt
    (finished: waiting for monitor prompt, in 0.00 seconds)
    (finished: sending monitor command: sendkey g, in 0.00 seconds)
    machine: sending monitor command: sendkey p
    machine: waiting for monitor prompt
    (finished: waiting for monitor prompt, in 0.00 seconds)
    (finished: sending monitor command: sendkey p, in 0.00 seconds)
    machine: sending monitor command: sendkey g
    machine: waiting for monitor prompt
    (finished: waiting for monitor prompt, in 0.00 seconds)
    (finished: sending monitor command: sendkey g, in 0.00 seconds)
    machine: sending monitor command: sendkey spc
    machine: waiting for monitor prompt
    (finished: waiting for monitor prompt, in 0.00 seconds)
    (finished: sending monitor command: sendkey spc, in 0.00 seconds)
    machine: sending monitor command: sendkey 0x0C
    machine: waiting for monitor prompt
    (finished: waiting for monitor prompt, in 0.00 seconds)
    (finished: sending monitor command: sendkey 0x0C, in 0.00 seconds)
    machine: sending monitor command: sendkey 0x0C
    machine: waiting for monitor prompt
    (finished: waiting for monitor prompt, in 0.00 seconds)
    (finished: sending monitor command: sendkey 0x0C, in 0.00 seconds)
    machine: sending monitor command: sendkey g
    machine: waiting for monitor prompt
    (finished: waiting for monitor prompt, in 0.00 seconds)
    (finished: sending monitor command: sendkey g, in 0.00 seconds)
    machine: sending monitor command: sendkey e
    machine: waiting for monitor prompt
    (finished: waiting for monitor prompt, in 0.00 seconds)
    (finished: sending monitor command: sendkey e, in 0.00 seconds)
    machine: sending monitor command: sendkey n
    machine: waiting for monitor prompt
    (finished: waiting for monitor prompt, in 0.00 seconds)
    (finished: sending monitor command: sendkey n, in 0.00 seconds)
    machine: sending monitor command: sendkey 0x0C
    machine: waiting for monitor prompt
    (finished: waiting for monitor prompt, in 0.00 seconds)
    (finished: sending monitor command: sendkey 0x0C, in 0.00 seconds)
    machine: sending monitor command: sendkey k
    machine: waiting for monitor prompt
    (finished: waiting for monitor prompt, in 0.00 seconds)
    (finished: sending monitor command: sendkey k, in 0.00 seconds)
    machine: sending monitor command: sendkey e
    machine: waiting for monitor prompt
    (finished: waiting for monitor prompt, in 0.00 seconds)
    (finished: sending monitor command: sendkey e, in 0.00 seconds)
    machine: sending monitor command: sendkey y
    machine: waiting for monitor prompt
    (finished: waiting for monitor prompt, in 0.00 seconds)
    (finished: sending monitor command: sendkey y, in 0.00 seconds)
    machine: sending monitor command: sendkey ret
    machine: waiting for monitor prompt
    (finished: waiting for monitor prompt, in 0.00 seconds)
    (finished: sending monitor command: sendkey ret, in 0.00 seconds)
    (finished: sending keys 'gpg --gen-key\n', in 0.15 seconds)

After:

    subtest: Can generate a PGP key
    machine: sending keys 'gpg --gen-key\n'
    (finished: sending keys 'gpg --gen-key\n', in 0.15 seconds)

0 files changed, 0 insertions, 0 deletions