git_mwindow_file_deregister() shouldn't return errors

As a function that appears to only be called on error paths, I don't
think it makes sense for it to return an error, or clobber the global
giterr. Note that no existing callsites actually check the return
code.

In my own application, there are errors where the real error ends
up being hidden, as git_mwindow_file_deregister() clobbers the
global giterr. I'm not sure this error is even relevant?
Merge pull request #1194 from libgit2/dont-clobber-errors

git_mwindow_file_deregister() shouldn't return errors