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.