Remove "set/show remotebaud" (deprecated) commands.

This patch removes support for the "set/show remotebaud" command,
which were deprecated in GDB 7.7, and should be now be removed
ahead of cutting the GDB 7.8 branch.

gdb/ChangeLog:

        * serial.c (_initialize_serial): Remove support for
        the "set remotebaud" and "show remotebaud" commands.
        * NEWS: Add entry documenting the removal of that command.

gdb/testsuite/ChangeLog:

        * config/monitor.exp (gdb_target_monitor): Replace use of
        "set remotebaud" by "set serial baud".