Don't call clear_quit_flag in command_handler

Don't call clear_quit_flag in command_handler

This just looks totally wrong to me, for completetly discarding a
user-requested Ctrl-C.  I can't think of why we'd want do this here.

Actually, I digged the history, and found out that this has been here
since at least 7b4ac7e1ed2c (gdb-2.4, the initial revision, 1988), at
a time were we had a top level setjmp/longjmp, long before that got
wrapped in throw_exception and friends, and this code was in an
explicit loop, with the quit_flag cleared on every iteration, before
executing a command...

gdb/ChangeLog:
2016-04-12  Pedro Alves  <palves@redhat.com>

	* event-top.c (command_handler): Don't call clear_quit_flag.