Remove some dead code from event-loop.c

This removes some dead code from event-loop.c.

This patch is from my old series to merge the gdb and gdbserver event
loops; but since it is just removing dead code, it seemed simple to
commit it separately.

gdb/ChangeLog
2020-02-12  Tom Tromey  <tom@tromey.com>

	* event-loop.c (event_data, gdb_event, event_handler_func):
	Remove.