Remove assertion over fetch refspec when explicitly specified

[rrei]

As we discussed in issue #396, the repo's fetch refspec is now only required if no explicit refspec was given as argument to Remote.fetch() or Remote.pull().

[Byron]

Thanks a lot for your contribution ! It's much appreciated, and if you need a release with it please let me know.

[rrei]

Glad I could help out, even if only a tiny bit :)
In my code I worked around this by using repo.git.fetch() directly, but maybe I'll refactor it later to use the proper Remote.fetch() method. So there's no need to do a release just for this ;)