Fix combine js command in utils.sh

* Remove obsolete ThemeHeartbeat.js file
* Overwrite bundle.js instead of appending to it

build/utils.sh

@@ -16,8 +16,7 @@ combine_javascript_sources() {
 			LightDMObjects.js \
 			Greeter.js \
 			GreeterConfig.js \
-			ThemeUtils.js \
+			ThemeUtils.js > "${MESON_SOURCE_ROOT}/src/gresource/js/bundle.js"
-			ThemeHeartbeat.js >> "${MESON_SOURCE_ROOT}/src/gresource/js/bundle.js"
 	}
 }