Revert "surfaceflinger: clear HWC layers in Layer::onRemoved"

Causing failures due to lack of clearHwcLayers in git_master and few 
other branches/builds. Looks like clearHwcLayers needs USE_HWC2 
to be defined, seems to be not defined in few branches/builds?

This reverts commit 06837418.

Change-Id: Ic0e3a99b45543503d5fa8a90f3569764ebe922e3