emacs: do not call notmuch show for non-inlinable parts

emacs: do not call notmuch show for non-inlinable parts

Before the change, there was a workaround to avoid notmuch show calls
for parts with application/* Content-Type.  But non-inlinable parts
are not limited to this Content-Type (e.g. mp3 files have audio/mpeg
Content-Type and are not inlinable).  For such parts
`notmuch-show-insert-part-*/*' handler is called which unconditionally
fetches contents for all parts.

The patch moves content fetching from `notmuch-show-insert-part-*/*'
to `notmuch-show-mm-display-part-inline' function after MIME inlinable
checks are done to avoid useless notmuch show calls.  The
application/* hack is no longer needed and removed.