Remove old Fort Mode from frontend
Part of ripping out Fort Mode from game schemes. This time for real.
- Remove the game modifier button "Fort Mode"
- Remove the scheme "Fort Mode"
- Remove some fort mode handling code
- Keep gfForts symbol to not mess up things too much
- Old schemes are still supported, the fortsmode parameter is just ignored
07f3bf8d98a3
Hedgewars.png had a resolution more than twice as high as required; also fixed some (mostly obsolete) code that also contained a typo that would cause a warning/note in clang