gnu: extremetuxracer: Update to 0.7.3.

* gnu/packages/games.scm (extremetuxracer): Update to 0.7.3.
[inputs]: Remove freetype, mesa, libice, libpng, sdl, sdl-mixer,
sdl-image, libsm, libunwind, libx11, libext, libxi, libxmu, libxt, tcl,
zlib; add sfml.
[arguments]: Remove field.