Clean up favicon url cache file location migration

:memo: Context

In reference to the TODO under the DefaultURLCacheFileManager. The original task was closed by the stale bot.

:checkered_flag: Requirements

• The location of the cache was moved in https://github.com/mozilla-mobile/firefox-ios/pull/13773
• We should remove the oldDirectory since it's been long enough
• Look if unit tests should be adjusted

:information_source: Reference person

@lmarceau

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