git-gui: Remove no longer used pull from remote code.

git-gui: Remove no longer used pull from remote code.

Because we aren't going to support single click pulling of changes from
an existing remote anytime in the near future, I'm moving the code which
used to perform that action.  Hopefully we'll be able to do something
like it in the near-future, but also support local branches just as
easily.

Signed-off-by: Shawn O. Pearce <redacted>