Revert "Add support for decoding base64."

This reverts commit 8b987ca5.

This change breaks several Windows bots
 - https://lab.llvm.org/buildbot/#/builders/123/builds/11371
 - https://lab.llvm.org/buildbot/#/builders/117/builds/7685
 - https://lab.llvm.org/buildbot/#/builders/42/builds/6077
 - https://lab.llvm.org/buildbot/#/builders/216/builds/6340