Makefile: fix cpp proto cleanup

The former find invocations were only operating inside ./pkg, but the
CPP protos are generated into c-deps/protos and c-deps/protoccl. Since
they have dedicated directories, just nuke those directories instead of
mucking around with find.

Release note: None

Makefile

@@ -987,14 +987,14 @@ $(GW_PROTOS_TARGET): $(PROTOC) $(GW_SERVER_PROTOS) $(GW_TS_PROTOS) $(GO_PROTOS)
 	touch $@
 
 $(CPP_PROTOS_TARGET): $(PROTOC) $(CPP_PROTOS)
-	$(FIND_RELEVANT) -type f \( -name '*.pb.h' -o -name '*.pb.cc' \) -exec rm {} +
+	rm -rf $(CPP_PROTO_ROOT)
 	mkdir -p $(CPP_PROTO_ROOT)
 	build/werror.sh $(PROTOC) -I$(PKG_ROOT):$(GOGO_PROTOBUF_PATH):$(PROTOBUF_PATH) --cpp_out=lite:$(CPP_PROTO_ROOT) $(CPP_PROTOS)
 	$(SED_INPLACE) -E '/gogoproto/d' $(CPP_HEADERS) $(CPP_SOURCES)
 	touch $@
 
 $(CPP_PROTOS_CCL_TARGET): $(PROTOC) $(CPP_PROTOS_CCL)
-	$(FIND_RELEVANT) -type f \( -name '*.pb.h' -o -name '*.pb.cc' \) -exec rm {} +
+	rm -rf $(CPP_PROTO_CCL_ROOT)
 	mkdir -p $(CPP_PROTO_CCL_ROOT)
 	build/werror.sh $(PROTOC) -I$(PKG_ROOT):$(GOGO_PROTOBUF_PATH):$(PROTOBUF_PATH) --cpp_out=lite:$(CPP_PROTO_CCL_ROOT) $(CPP_PROTOS_CCL)
 	$(SED_INPLACE) -E '/gogoproto/d' $(CPP_HEADERS_CCL) $(CPP_SOURCES_CCL)