Revert "Moved browser specific settings into manifest"

This reverts commit 047432af03.

firefox_manifest-extra.json

@@ -0,0 +1,8 @@
+{
+  "browser_specific_settings": {
+    "gecko": {
+      "id": "sponsorBlocker@ajay.app",
+      "strict_min_version": "57.0"
+    }
+  }
+}