Remove unused variables, fix #5161

[kimden]

Previously some of them were commented in one file without realizing they are extern in another file.

Agreement

By creating a pull request in stk-code, you hereby agree to dual-license your contribution as
GNU General Public License version 3 or any later version and
Mozilla Public License version 2 or any later version.

This includes your previous contribution(s) under the same name of contributor.

Keep the above statement in the pull request comment for agreement.