Bug 1240789 - Remove head+tail in Marionette xpcshell manifest; r=automatedtester

Was originally there to work around a bug in the B2G on-device test
runner.  Can now safely be removed after bug 1240723 was landed.

MozReview-Commit-ID: EZGfvYcP9ER

--HG--
extra : rebase_source : c2dc099b127b4bbd5988c386d15de72a3ab0bf40

testing/marionette/unit.ini

@@ -5,9 +5,7 @@
 # xpcshell unit tests for Marionette
 
 [DEFAULT]
-head =
-tail =
-skip-if = appname == "thunderbird"  # Thunderbird does not use marionette
+skip-if = appname == "thunderbird"
 
 [test_error.js]
 [test_message.js]