surfaceflinger: clear HWC layers in Layer::onRemoved v2

Explicitly destroy HWC layers in Layer::onRemoved rather than doing
that implicitly in the destructor.  The layer may be destructed by
any thread that holds a sp<Layer>, but only the main thread should
talk to the composer.

v2 fixes build breakage for non-HWC2 targets.

Bug: 38151478
Bug: 37978067
Test: manual
Change-Id: I8858fc46de1fc6facded219740b931c6486e317e