pullrequest overview: remove custom handling of 'my pullrequests'

Re-use the same displaying code for the 'my pullrequests' overview as for
the repository pullrequests overview. Remove the now unused CSS styling as
well.

This removes the 'delete' button on the author's pullrequests. One could
argue whether the pullrequest overview is the right place for this button,
while it does not appear on the pull request page itself.