Introduce 'enum remove_bp_reason'

Makes the code more obvious.

gdb/ChangeLog:
2016-08-10  Pedro Alves  <palves@redhat.com>

	PR gdb/19187
	* breakpoint.c (insertion_state_t): Delete.
	(enum remove_bp_reason): New.
	(detach_breakpoints, remove_breakpoint_1, remove_breakpoint):
	Adjust to use enum remove_bp_reason instead of insertion_state_t.