export doesn't allow overwriting current document (?)

https://bugzilla.redhat.com/show_bug.cgi?id=1673198 points out that if
you import a pdf document and then try and re-export that pdf to
overwrite the original, the overwrite is not allowed. (Though there is
no such general problem with save/save-as to overwrite the original
document)

This is deliberate thing since 
https://bz.apache.org/ooo/show_bug.cgi?id=81437 "Do not allow to export
to the same file.", but without an indication if this is due to some
technical glitch if you do, or for some human interaction guideline
around exporting.

With https://gerrit.libreoffice.org/#/c/67491 I'll revert that
restriction out in master and see how that works out.