Remove minigzip.c which causes error with emscripten build

minigzip.c uses non-standard functions such as _unlock and _nofile which causes problem with emscripten 1.14.1.
minigzip.c is not a part of gzip and it is only a test example. It wasn't originally included for building anyway. commit abf2a changes cmake/third-party/zlib/CMakeLists.txt to use glob which includes the erroneous file.

In the future, 1.2.8 can be considered, which moves all extra files away from the root directory.