Follow-up to https://github.com/mozilla/web-ext/pull/2306/files#r698444621

web-ext doesn't need to ask for the READ_EXTERNAL_STORAGE permission on Android, because the profile/add-on is now stored in /data/local/tmp instead of /sdcard: https://github.com/mozilla/web-ext/blame/c884696fd3ea496c67959337f98d4e39bdb08871/src/util/adb.js#L14 (#2200).

Let's remove the code that does it to simplify the implementation.