Remove obsolete proguard flags

R8 now warns:
Warning in ./build/make/core/proguard_basic_keeps.flags:
  Ignoring option: -optimizations

Remove the flags, they are not used by R8.

Fixes: 123401079
Test: m checkbuild
Change-Id: I8b0c558be8bcf943652dddb71802ddc145ae42c1