Removes the stale workaround put in place that disabled zlib altogether for cross-compiling before we had CMAKE_FIND_ROOT_PATH set properly in all of our toolchain files. Now that we're seeing the find-root properly we need to remove the workaround to allow finding a target zlib when cross-compiling.
Tested manually on an aarch64 cross-compilation.
The core of i#2285 was fixed earlier by PR #2551; this finishes off the issue.
Fixes #2285 (closed)