surfaceflinger: remove TARGET_USES_HWC2

Remove TARGET_USES_HWC2 and assume it to be true.  Remove the unused
SurfaceFlinger_hwc1.cpp and DisplayHardware/HWComposer_hwc1.cpp.

This is expected to break some devices.  All affected devices should
add

  PRODUCT_PACKAGES += android.hardware.graphics.composer@2.1-impl

to their device.mk and update their manifest.xml.  In the worse
case, we want to fork frameworks/native/services/surfaceflinger
before this change to
frameworks/native/services/surfaceflinger_hwc1.

Test: boots taimen and hikey960
Change-Id: I27ddd4c6c8af3c39cda99d94002d179486575b28