Remove the attachment text extraction feature.

This feature is currently implemented by hooking into the virus-scanning process to do the actual text extraction.  As well as significantly slowing down the virus scanning process, this has caused issues where tika gets wedged, and then blocks all virus scanning.

Until we have the time to implement this feature properly, the benefit it provides to our search index isn't worth the operational pain it's causing us.  This PR therefore removes the feature (while leaving it in place for HTML attachments because they don't have the same external processing requirements for this).

Note: I plan to squash these 2 commits down before merging, but I've left them separate for now to aid reviewing.