Removed reference to main_for_testing.cc in firestore (#546)

Author: Alex Ames

firestore/CMakeLists.txt

@@ -39,7 +39,6 @@ set(common_SRCS
     src/common/listener_registration.cc
     src/common/load_bundle_task_progress.cc
     src/common/macros.h
-    src/common/main_for_testing_build.cc
     src/common/query.cc
     src/common/query_snapshot.cc
     src/common/set_options.cc