surfaceflinger: do not check HWC2::Capability::SkipValidate

HWC2::Capability::SkipValidate is not defined in HIDL.  We cannot
check against it.

Besides, presentOrValidate is equivalent to validate in HIDL when
the underlying hwcomposer2 implementation does not support the cap.
There is not need to check against it.

Bug: 70407085
Test: boots
Change-Id: Ib0c11e99ce3389ad555605de18e194d9b51b2bc0