licensing: fix zlib license translation
The file was mangled, and something about "teeworlds" was inserted
into the line for zlib. The spdx database doesn't list "teeworlds
license", so let's just remove that part.
Both [1] and [2] agree that "zlib" and "zlib license" are the same thing.
[1] https://spdx.org/licenses/Zlib
[2] https://fedoraproject.org/wiki/Licensing:Main#Good_Licenses
Fixes #61.