helper/command: silent debug msg on command register/unregister

helper/command: silent debug msg on command register/unregister

Commit e216186fab59 ("helper/command: register full-name commands
in jim") and commit a7d68878e4ba ("helper/command: unregister
commands through their full-name") introduce a LOG_DEBUG() message
each for command registration and unregistration.
The messages above are quite noisy and pollute the log when
debug_level is 3 or higher.
They can be useful to debug the command registration logic, but
for the other debug activities on OpenOCD are just noisy.
Already commit a03ac1ba3087 ("helper/command: disable logging of
registered commands [RFC]") was merged to silent the first case
that is now back with additional logs.

Silent both log messages.
Use 'if (false)' to silent them, making easy to re-enable it when
or if someone needs it.

Change-Id: Id8a067e60e822d4ecbddcb036d081298f7e6181f
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/6220
Tested-by: jenkins