No issue: remove unused FirefoxTestRunner.

It would be used for on device tests but it doesn't seem to hooked up
currently.

app/src/androidTest/java/org/mozilla/fenix/FirefoxTestApplication.kt

@@ -1,7 +0,0 @@
-/* This Source Code Form is subject to the terms of the Mozilla Public
- * License, v. 2.0. If a copy of the MPL was not distributed with this
- * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
-
-package org.mozilla.fenix
-
-class FirefoxTestApplication : HomeActivity()

app/src/androidTest/java/org/mozilla/fenix/FirefoxTestRunner.kt

@@ -1,16 +0,0 @@
-/* This Source Code Form is subject to the terms of the Mozilla Public
- * License, v. 2.0. If a copy of the MPL was not distributed with this
- * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
-
-package org.mozilla.fenix
-
-import android.app.Application
-import android.content.Context
-import androidx.test.runner.AndroidJUnitRunner
-
-class FirefoxTestRunner : AndroidJUnitRunner() {
-
-    override fun newApplication(cl: ClassLoader?, className: String?, context: Context?): Application {
-        return super.newApplication(cl, FirefoxTestApplication::class.java.name, context)
-    }
-}