Remove __ASSUME_F_GETOWN_EX.

This patch removes __ASSUME_F_GETOWN_EX now it can be assumed to be
true unconditionally.

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

	* sysdeps/unix/sysv/linux/kernel-features.h
	(__ASSUME_F_GETOWN_EX): Remove macro.
	* sysdeps/unix/sysv/linux/fcntl.c: Do not include
	<kernel-features.h>.
	(miss_F_GETOWN_EX): Remove variable or macro.
	(do_fcntl): Do not check miss_F_GETOWN_EX.
	(do_fcntl) [!__ASSUME_F_GETOWN_EX]: Remove conditional code.