Drop libSDL useflag dependencies for the client.

Technically not needed for the steam client due to aggressive enforcement of the bundled libSDL2-2.0.so in ~/.local/share/Steam/ubuntu12_32/. An appropreate line should be added in games-util/steam-games-meta if we have incomplete coverage from this change.
Merge pull request #115 from Tele42/patch-2

Drop libSDL useflag dependency for the client.