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 the entire virus scanning queue.

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 - we just run the body through Govspeak).