surfaceflinger: remove USE_HWC2

Remove USE_HWC2 and assume it to be true.  Remove all !USE_HWC2
code.

This also fixes a compile error in tests/fakehwc, which never
defines USE_HWC2.

Test: boots taimen and hikey960
Change-Id: Ie6d2fcf884aa6fc715ab0ce19a1e0f7c8acf734c