Remove unnecessary platform.architecture() call.  While it's true this workaround is only needed for 64-bit OSX systems, that comprises the vast majority of OSX systems in use.  The workaround does not add significant overhead for 32-bit systems (and is not even likely to be encountered since most 32-bit systems won't be able to handle such large arrays to begin with).
Merge pull request #2345 from embray/issue-2345

Remove platform.architecture() in FITS _array_to_file to improve speed on Mac