Remove locking from `Transaction`.

Multi-threaded execution of transaction has never been fully
supported, so remove the unnecessary locking as it is pure
overhead. (And implementing it would require a lot of effort
on transactions interfering with each other.)

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Bug: 181943478
Change-Id: Ibdbc335738d34ea1398f50f0cc763f3b3f5838b9
4 files changed