remove code causing conversion of text/x-tiddlywiki type tiddlers to tex...

...t/vnd.tiddlywiki

When using nodejs, saved tiddlers of type text/vnd.tiddlywiki get saved with type text/vnd.tiddlywiki.
Removing changing the type form text/vnd.tiddlywiki to null stop this, but maybe there is an underlying logical problem with tiddlyweb?