gnu: supertuxkart: Simplify unbundling and unbundle enet.

* gnu/packages/games.scm (supertuxkart)[source](snippet): Delete "lib/enet".
Remove "CMakeList.txt" substitution.
[arguments]<#:configure-flags>: Add "-DUSER_SYSTEM_GLEW=TRUE" and
"-DUSE_SYSTEM_ENET=TRUE".
<#:phases>: Delete.

Signed-off-by: Christopher Baines <mail@cbaines.net>