Remove the $wgHandheldStyle config variable entirely

Added back in 2008 (dc821ec1 / r38116), it was only ever implemented
for Monobook and didn't do anything in other skins.

The only use-case given was using Chick's CSS for mobile devices with
Monobook being the default skin - and now Chick is no more and
Monobook isn't the default.

It was also used to decide whether to load styles from
MediaWiki:Handheld.css (they were loaded if it was set to anything).
That message has been removed as well.

Bug: 47504
Change-Id: Ia8d79b4a637a227eb1879fbb874173fdd9bd6d99
Merge "Remove the $wgHandheldStyle config variable entirely"