linux/clock_settime: remove unnecessary vDSO definitions

These definitions were added back before __ASSUME_POSIX_CPU_TIMERS
was removed.  There used to be a vsyscall to clock_getres() in
maybe_syscall_settime_cpu(), but that function was removed in commit
26889eac.  The presence of the vsyscall definitions means that platforms
that don't provide clock_getres as a vsyscall hit a symbol redefinition
warning in this file, becoming fatal with -Werror.  Removing the
vsyscall definitions is the obvious fix.

No change to generated code on x86_64.