selftests: vDSO: vdso_test_abi: Drop clock availability tests

The test uses the kselftest.h framework and declares in its testplan to
always execute 16 testcases. If any of the clockids were not available,
the testplan would not be satisfied anymore and the test would fail.
Apparently that never happened, so the clockids are always available.

Remove the pointless checks.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250812-vdso-tests-fixes-v2-4-90f499dd35f8@linutronix.de