Finalize converting resources.txt into immutable resources

With hindsight, I revised decisions made earlier during
this development cycle:

Un-redirectable scriptlets have been removed from
/web_accessible_resources and instead put in the new
/assets/resources/scriptlets.js, which contains all
scriptlets used for web page injection purpose.

uBO will no longer fetch a remote version of built-in
resources.

Advanced setting `userResourcesLocation` will still be
honoured by uBO, and if set, will be fetched every
time at least one asset is updated.

assets/assets.json

@@ -19,10 +19,7 @@
 	"ublock-resources": {
 		"content": "internal",
 		"updateAfter": 7,
-		"contentURL": [
-			"https://raw.githubusercontent.com/uBlockOrigin/uAssets/master/filters/resources.txt",
-			"assets/ublock/resources.txt"
-		]
+		"contentURL": "assets/ublock/resources.txt"
 	},
 	"ublock-filters": {
 		"content": "filters",