CMake: reverted some in `cmake/environmentchecks.cmake`.
CMake: reverted some in `cmake/environmentchecks.cmake`.

For some reasons, this was added, but has nothing to do with the issue at hand.

nickerso/libcellml,cellml/libcellml,nickerso/libcellml,cellml/libcellml,hsorby/libcellml,hsorby/libcellml,nickerso/libcellml,cellml/libcellml,cellml/libcellml,hsorby/libcellml,nickerso/libcellml,hsorby/libcellml