.gitconfig: Remove global remote branch

This causes duplicate remotes to be created, potentially leading Git to throw errors when `push`ing.

Ref. https://github.com/mathiasbynens/dotfiles/commit/3803a98eaf001e2b96cb6513264c73a845035093
.gitconfig: change push.default to `upstream`

* `tracking` is depracated
* Remove remote.origin.push HEAD since it’s implied with push.default