Remove __ASSUME_COMPLETE_READV_WRITEV.

Remove __ASSUME_COMPLETE_READV_WRITEV.

This patch removes the __ASSUME_COMPLETE_READV_WRITEV
kernel-features.h macro, now that it can be unconditionally assumed to
be true.  (The relevant kernel feature was added some time between 2.0
and 2.2, and this macro is only used in sysdeps/unix/sysv/linux/.)

Tested x86_64 that the disassembly of installed shared libraries is
unchanged by this patch.

	* sysdeps/unix/sysv/linux/kernel-features.h
	(__ASSUME_COMPLETE_READV_WRITEV): Remove macro.
	* sysdeps/unix/sysv/linux/readv.c: Do not include
	<kernel-features.h>.
	[!__ASSUME_COMPLETE_READV_WRITEV]: Remove conditional code.
	[!UIO_FASTIOV] (UIO_FASTIOV): Remove macro.
	(__libc_readv) [__ASSUME_COMPLETE_READV_WRITEV]: Make code
	unconditional.
	(__libc_readv) [!__ASSUME_COMPLETE_READV_WRITEV]: Remove
	conditional code.
	* sysdeps/unix/sysv/linux/writev.c: Do not include
	<kernel-features.h>.
	[!__ASSUME_COMPLETE_READV_WRITEV]: Remove conditional code.
	[!UIO_FASTIOV] (UIO_FASTIOV): Remove macro.
	(__libc_writev) [__ASSUME_COMPLETE_READV_WRITEV]: Make code
	unconditional.
	(__libc_writev) [!__ASSUME_COMPLETE_READV_WRITEV]: Remove
	conditional code.