Remove BOARD_OVERRIDE_RS_CPU_VARIANT_*

These variables were only being used on flounder and dragon.  Dragon
was copied from flounder, and was unnecessary as dragon has the same
Cortex A53/A57 CPUs as most other Android devices.  Flounder is also
unnecessary, as LLVM is returning "generic" for A53/A57, and will
return "generic" for denver.

Test: mmma -j frameworks/compile/libbcc
Change-Id: Ic52e2dca219b4714488a9412aedcb0ecbea83540