[microblaze] Remove __ASSUME_FUTIMESAT.

[microblaze] Remove __ASSUME_FUTIMESAT.

MicroBlaze has a special version of futimesat.c because it gained the
futimesat syscall later than other non-asm-generic architectures.  Now
the minimum kernel is recent enough that this syscall can always be
assumed to be present for MicroBlaze, so this patch removes the
special version and the __ASSUME_FUTIMESAT macro, resulting in the
sysdeps/unix/sysv/linux/futimesat.c version being used.

Untested.

	* sysdeps/unix/sysv/linux/microblaze/kernel-features.h
	(__ASSUME_FUTIMESAT): Remove macro.
	* sysdeps/unix/sysv/linux/microblaze/futimesat.c: Remove file.