Remove two ReadBarrierMarkRegX entrypoints.

As entry points ReadBarrierMarkReg30 and
ReadBarrierMarkReg31 are undefined on all architectures
supporting the read barrier configuration (ARM, ARM64, x86
and x86-64), remove them from the entry point list.

Test: ART host and target (ARM, ARM64) tests.
Bug: 29506760
Bug: 12687968
Change-Id: I500626e54f00aebfc095b4ef5f81b49fa43f7768