[vim] Fix color scheme background with GitGutter

Having

    highlight clear SignColumn

with

    highlight GitGutterAdd ctermfg=green
    highlight GitGutterChange ctermfg=yellow
    highlight GitGutterDelete ctermfg=red
    highlight GitGutterChangeDelete ctermfg=yellow

actually negates the first option of clearing the SignColumn. As a
result, we're not adhering to the color scheme's background color after
all.

By commenting the last four lines of the commit, we fix the previous
commit by actually using the color scheme's background color.