Remove __ASSUME_GETDENTS64_SYSCALL.

This patch removes the __ASSUME_GETDENTS64_SYSCALL macro, as its
definition is constant given the new kernel version requirements (and
was constant anyway before those requirements except for MIPS n32).

Note that the "#ifdef __NR_getdents64" conditional *is* still needed,
because MIPS n64 only has the getdents syscall (being a 64-bit ABI,
that syscall is 64-bit; the difference between the two on 64-bit
architectures is where d_type goes).  If MIPS n64 were to gain the
getdents64 syscall and we wanted to use it conditionally on the kernel
version at runtime we'd have to revert this patch, but I think that's
unlikely (and in any case, we could follow the simpler approach of
undefining __NR_getdents64 if the syscall can't be assumed, just like
we do for accept4 / recvmmsg / sendmmsg syscalls on architectures
where socketcall support came first).

Most of the getdents.c changes are reindentation.

Tested for x86_64 and x86 that installed stripped shared libraries are
unchanged by the patch.

	* sysdeps/unix/sysv/linux/kernel-features.h
	(__ASSUME_GETDENTS64_SYSCALL): Remove macro.
	* sysdeps/unix/sysv/linux/getdents.c
	[!__ASSUME_GETDENTS64_SYSCALL]: Remove conditional code.
	[!have_no_getdents64_defined]: Likewise.
	(__GETDENTS): Remove __have_no_getdents64 conditional.

sysdeps/unix/sysv/linux/kernel-features.h

@@ -48,10 +48,6 @@
    and still does not have a 64-bit inode field.  */
 #define __ASSUME_ST_INO_64_BIT		1
 
-/* The getdents64 syscall was introduced in 2.4.0-test7 (but later for
-   MIPS n32).  */
-#define __ASSUME_GETDENTS64_SYSCALL	1
-
 /* The statfs64 syscalls are available in 2.5.74 (but not for alpha).  */
 #define __ASSUME_STATFS64	1