SF: Remove dead code from Scheduler

While looking into some other Scheduler changes, I noticed that both
Scheduler::mRenderEventConnection and
Scheduler::mLastCompositeEventConnection are initialized but otherwise
unused. Scheduler::getEventConnection() is never called to retrieve
either value.

As there doesn't seem to be any use for them, let's remove them.

This change also required removing test fixture setup from the unit
tests, as the tests worked to install test double implementations.

Test: atest libsurfaceflinger_unittest
Bug: None
Flag: EXEMPT dead code

Change-Id: Ic004df96dd5fe3dadd0cd1455cd85506736a1875