emacs: Remove broken `notmuch-get-bodypart-content' API

emacs: Remove broken `notmuch-get-bodypart-content' API

`notmuch-get-bodypart-content' could do two very different things,
depending on conditions: for text/* parts other than text/html, it
would return the part content as a multibyte Lisp string *after*
charset conversion, while for other parts (including text/html), it
would return binary part content without charset conversion.

This commit completes the split of `notmuch-get-bodypart-content' into
two different and explicit APIs: `notmuch-get-bodypart-binary' and
`notmuch-get-bodypart-text'.  It updates all callers to use one or the
other depending on what's appropriate.