libbinder: remove __ANDROID_APEX__ stability diff

These were setup because the media APEX module was using vendor variants
of libbinder on the system partition. However, now that this build
dependency is cleaned up, we can remove the #ifdefs and make this a
build error instead of a runtime error.

Test: binderStabilityTest
Fixes: 179906909
Change-Id: If05cf3fa0851866d7113f6e8f9705891672ad369