include: posix: remove an unreachable clockid_t definition

The clockid_t definition in question is not reachable anymore
because at least one of _CLOCKID_T_DECLARED and __clockid_t_defined
macros is already defined by that point, always making the check fail.

Signed-off-by: Ilya Tagunov <Ilya.Tagunov@synopsys.com>