Remove obsolete proguard rule for unconditionally keep specific classes

This should be covered by the keep rules that aapt outputs

Local testing shows significant savings with this on SystemUIGoogle:
500K on dex
400K on resources (in proto format, could be more)

Bug: 287398085
Test: Test existing
Flag: NONE
Change-Id: I0bf358d57985a8c591ed59847f45c03b7771473b