Remove stale session cleanups during tests

Fixes spurious mockito WrongTypeOfReturnValue errors during unit tests.
The errors were most likely due to a stale instance of Context sticking
around in the static Log class across test cases. This change makes Log
no longer try to access its instance of Context during testing.

Bug: 27484228
Change-Id: I2985e7c29722b27d4e456fe58557deded68f9086