[BOLT] Remove threaded EliminateUnreachableBlock version

Summary:
EliminateUnreachableBlocks has a data race because it depends
on BinaryContext::computeCodeSize. computeCodeSize supports independent
Emitters, enabling a lock-free execution. Unfortunately, that is almost
as expensive as the lock. Removing the boilerplate code for
parallellization of this pass turned out to be the best alternative: no
races and slightly better execution time for HHVM.

(cherry picked from FBD24716250)