delete duplicated set_team fcn

game_display and display versions of set_team were identical, in
header and in implementation, so we simply delete the game_display
one, as a strict refactor.
fixup segfaults in replay viewer

After this commit e4eb0a3ede83ee338994708ebd48257627b47242 the
replay viewer would segfault during prestart events. However, it
turns out that nothing in the replay viewer, or in that commit,
was directly causing the segault. Instead, the display object
was holding dangling pointers for no reason, when it could have.
been simply computing the correct value (very cheaply). We clean
up the code in the display objects to fix the segfault.
Merge branch 'master' of git://github.com/wesnoth/wesnoth