Remove __ASSUME_SIGNALFD4.

Current Linux kernel version requirements mean the signalfd4 syscall
can always be assumed to be available.  This patch removes
__ASSUME_SIGNALFD4 and associated conditionals.

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

	* sysdeps/unix/sysv/linux/kernel-features.h (__ASSUME_SIGNALFD4):
	Remove macro.
	* sysdeps/unix/sysv/linux/signalfd.c: Do not include
	<kernel-features.h>.
	(signalfd) [__NR_signalfd4]: Make code unconditional.
	(signalfd) [!__ASSUME_SIGNALFD4]: Remove conditional code.