Remove __ASSUME_ADJ_OFFSET_SS_READ.

This patch removes the __ASSUME_ADJ_OFFSET_SS_READ macro (and
conditionals on whether ADJ_OFFSET_SS_READ is defined), now it can be
unconditionally assumed to be true and ADJ_OFFSET_SS_READ can be
assumed to be defined.

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

	* sysdeps/unix/sysv/linux/kernel-features.h
	(__ASSUME_ADJ_OFFSET_SS_READ): Remove macro.
	* sysdeps/unix/sysv/linux/adjtime.c (ADJTIME)
	[ADJ_OFFSET_SS_READ]: Make code unconditional.
	(ADJTIME) [!ADJ_OFFSET_SS_READ]: Remove conditional code.