Remove fail() override in Mocha

I'm not sure what this was for, but at least with an async test function
it seems to be causing spurious "the string 'x' was thrown, throw an
Error :)" messages that hide the real error.

test/content/runtests.js

@@ -161,21 +161,6 @@ function Reporter(runner) {
 	});
 }
 
-// Monkey-patch Mocha to check instanceof Error using compartment-local
-// Error object
-Mocha.Runner.prototype.fail = function(test, err){
-	++this.failures;
-	test.state = 'failed';
-
-	if ('string' == typeof err) {
-		err = new Error('the string "' + err + '" was thrown, throw an Error :)');
-	} else if (!(err instanceof Components.utils.getGlobalForObject(err).Error)) {
-		err = new Error('the ' + Mocha.utils.type(err) + ' ' + Mocha.utils.stringify(err) + ' was thrown, throw an Error :)');
-	}
-
-	this.emit('fail', test, err);
-};
-
 // Setup Mocha
 mocha.setup({
 	ui: "bdd",