Remove the hardwired runner from the MMS test.

This line has to be removed in order to successfully launch the mms tests.
diff --git a/testrunner/test_defs.xml b/testrunner/test_defs.xml
index dd56db1..e3dacac 100644
--- a/testrunner/test_defs.xml
+++ b/testrunner/test_defs.xml
@@ -239,7 +239,6 @@
 <test name="mms"
     build_path="packages/apps/Mms"
     package="com.android.mms.tests"
-    runner="com.android.mms.ui.MMSInstrumentationTestRunner"
     coverage_target="Mms" />
 
 <test name="mmslaunch"