Remove __ASSUME_STATFS_F_FLAGS.

Now that 3.2 is the minimum Linux kernel version for glibc, this patch
removes __ASSUME_STATFS_F_FLAGS and associated conditional code.

Tested for x86_64.

	* sysdeps/unix/sysv/linux/kernel-features.h
	(__ASSUME_STATFS_F_FLAGS): Remove macro.
	* sysdeps/unix/sysv/linux/internal_statvfs.c
	[!__ASSUME_STATFS_F_FLAGS]: Remove conditional code.