Remove RenderEngine::flush from latchBuffer()

Fence fd will be owned by SurfaceFlinger to be propagated down to
latchBuffer for merging sync fences. We can't store the fd at the
Layer-level & flush on every draw() invocation for some reason (I don't
fully understand what the gl driver does under the hood, but from what
it looks like file descriptors are reused when the command stream is
flushed too often which causes a memory leak when a buffer's sync fence
is merged), so we'll let SF backend store the flush fence for the
previous frame.

Bug: 116277151
Change-Id: I7901b0178aa0f11505650bf5e1df6f085a5d93bf
Test: SurfaceFlinger_test, libsurfaceflinger_unittest, go/wm-smoke