Unflag apex_signature_permission_allowlist_enabled

Since it is trivial, isn't exported or used in tests, and 24Q3 has been
released.

Note that this is a different flag than
signature_permission_allowlist_enabled which we can't remove yet.

Bug: 308573169
Fixes: 346561120
Flag: EXEMPT removing apex_signature_permission_allowlist_enabled
Test: presubmit
Change-Id: I721f74cca091954f3dab41dd5bed86b9583da66e