emacs: Simplify subjects more aggressively.

Remove 're: ' or 'Re: ' from anywhere within a subject line rather
than just at the beginning. This is to accommodate threads where a
mailing list sometimes inserts a subject prefix.

For example, if a thread has the subjects:

    [Orgmode] org-indent, org-inlinetask: patches on github
    Re: [Orgmode] org-indent, org-inlinetask: patches on github
    [Orgmode] Re: org-indent, org-inlinetask: patches on github

the last of these would not have been considered the same and would
therefore have been shown.

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