remove errorprone/processor classpath from r8 flags

This classpath includes compile-time-only jars that may conflict with program/library jars referenced by R8 and used by the target.

Test: go test ./java
Test: m && inspect output for r8 warnings
Bug: 242088131
Change-Id: Id4db3c9482e78cdcc0c9d37b9a77eaf94ac95cb3