gdbarch_breakpoint_from_pc doesn't return NULL

gdbarch_breakpoint_from_pc doesn't return NULL except for
ia64_breakpoint_from_pc, and we checked its return value in three
places. In microblaze_linux_memory_remove_breakpoint and
ppc_linux_memory_remove_breakpoint, gdbarch_breakpoint_from_pc never
returns NULL, so we can remove the NULL checking.  In
default_memory_insert_breakpoint, gdbarch_breakpoint_from_pc can't
returns NULL too because ia64 defines its own memory_insert_breakpoint.

gdb:

2016-11-03  Yao Qi  <yao.qi@linaro.org>

	* mem-break.c (default_memory_insert_breakpoint): Don't check
	'bp' against NULL.
	* microblaze-linux-tdep.c (microblaze_linux_memory_remove_breakpoint):
	Likewise.
	* ppc-linux-tdep.c (ppc_linux_memory_remove_breakpoint): Likewise.