Unset USE_GOMA when using RBE.

Test: Ran rbesetup.sh after setting USE_GOMA
Change-Id: Idf6c1b4f7591aed96d5365e461260060004d4494
diff --git a/rbesetup.sh b/rbesetup.sh
index adcf081..724ad7d 100644
--- a/rbesetup.sh
+++ b/rbesetup.sh
@@ -36,6 +36,7 @@
 # This function sets RBE specific environment variables needed for the build to
 # executed by RBE. This file should be sourced once per checkout of Android code.
 function _set_rbe_vars() {
+  unset USE_GOMA
   export USE_RBE="true"
   export RBE_CXX_EXEC_STRATEGY="remote_local_fallback"
   export RBE_JAVAC=1