Remove the $wgHandheldForIPhone config variable entirely

It only really made sense in pair with $wgHandheldStyle, and that has
been removed in Ia8d79b4a.

Remove irrelevant tests, adjust still relevant ones.

Change-Id: I7c24128f7b148d0244538ad95bb60bf09ec4b5cb
Merge "Remove the $wgHandheldForIPhone config variable entirely"