libbinder: remove unused IPCThreadState::process

This function isn't used, and ProcessState is a global object.

Future consideration:
- would it be worth the saved memory in thread local storage to drop
  IPCThreadState::mProcess and reference the global?

Bug: N/A
Test: N/A
Change-Id: I984f57827d3ade423b24132dd5b5f18028dc2a1d