Remove libsurfaceflinger_unittest from root TEST_MAPPING

libsurfaceflinger_unittest has some flaky tests, and being run for all
frameworks/native changes means submitters are having to rerun
presubmit.

Bug: 186417847
Test: None (presubmit only)
Change-Id: Ie1f9542f8304493cdc40aec00e374a88cef42853
Merged-In: I99f9fdf5e75dae686f958e97e9685d1a00166877