Remove old configuration

This should have been removed in 662ae39.
Remove binding of undefined prefix map
Create a prefix-map for git-related functions
Add command magit-show-commit-under-point
Add command magit-show-project-commit-under-point
Add command magit-stage-file-intent
Expand magit-auto-commit docstring
Add command magit-commit-extend-all
Disable magit-backup-mode

As of Magit c268e7f, backups won't occur if the mode is disabled.
Set org-gnus-prefer-web-links
Use notmuch Org links for local mail