LoongArch: vdso: Remove timekeeper includes

LoongArch: vdso: Remove timekeeper includes

Since the generic VDSO clock mode storage is used, this header file is
unused and can be removed.

This avoids including a non-VDSO header while building the VDSO,
which can lead to compilation errors.

Also drop the comment which is out of date and in the wrong place.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20241010-vdso-generic-arch_update_vsyscall-v1-8-7fe5a3ea4382@linutronix.de