gdbserver: remove handling of the 'L' tracepoint action

gdbserver: remove handling of the 'L' tracepoint action

Now that static tracepoint support is removed from gdbserver, it makes
sense to remove handling of the 'L' tracepoint action, too.  The code
that checks received actions already has a default case that tolerates
unrecognized actions:

        default:
          trace_debug ("unknown trace action '%c', ignoring...", *act);

In case 'L' is unexpectedly received, we would at least be able to see
this in the logs.

Approved-By: Simon Marchi <simon.marchi@efficios.com>