Remove AmigaOS support.

It is unknown whether the platform still works with the current Wesnoth
code. To ease the SDL 2.0 transition support is removed. If a maintainer
steps up support can be added again.
Remove BeOS support.

It is unknown whether the platform still works with the current Wesnoth
code. To ease the SDL 2.0 transition support is removed. If a maintaine
steps up support can be added again.
Remove OS/2 support.

It is unknown whether the platform still works with the current Wesnoth
code. To ease the SDL 2.0 transition support is removed. If a maintainer
steps up support can be added again.
Merge branch 'remove_old_platforms'

Removes support to ease the transition to SDL2.