This patch removes the "Kerning Style" option for UI widget font
drawing and uses only the current default of "Fitted", since the other
option of "Unfitted" is just the result of truncation errors.

In Preferences / Themes / Text Style we give users two options for "Kerning Style". But as shown here there is really only one kerning style for any font. Twelve years ago it was noticed that our kerning was suboptimal (caused by truncation of the ideal values, rather than rounding), but rather than fix this we ended up with a set of options. The original, shown as "Unfitted" remained even though the fixed option "Fitted" became default. This option created a lot of code complication. Removing this bad option will not only simplify our code, but also make it more efficient, and open us up for further optimization.

This patch is basically a nice first step as it minimally removes the option without further refactoring. I will submit further patches the makes further changes because I'd rather keep these things a bit more atomic and easier to review.

First, everything is made to run with FT_KERNING_DEFAULT by just altering BLF_KERNING_VARS so that _kern_mode is always that value.

Then the user-facing kerning options are removed, and so the "kerning" member of uiFontStyle is removed.

Afterward there is nothing more than removing lots of bits of code that has to set FT_KERNING_DEFAULT on if it is not on, then off again, all unnecessary now.

Note there is no versioning here as I couldn't find a situation where it was needed, but I could be wrong.