Redundant executable check for native library

When loading the library from the temporary directory it is checked (introduced in e7a7547d2268cf1286bf314ab929ceda7a345e64) if executable permission bits can be applied in `Platform.canExecuteExecutable`. The check fails in a sandboxed environment on macOS where `Files.isExecutable()` always returns `false` (possibly a bug). Nevertheless, at least on macOS the dynamic library must not be executable to be successfully loaded subsequently. The check could be omitted alltogether. Don't know if this is a requirement on other platforms.