nvim: set EDITOR variable only if installed

Since git core.editor was set in the initial commit, I suspect I wasn't
aware of $EDITOR in the environment. So it's safe to delete
git/gitconfig's core.editor and take it from $EDITOR by default, which
actually checks if nvim is installed.

for git's diff.tool and merge.tool, I don't see a possibility to have a
conditional switch and fallback to vim. I don't expect to work on
another workstation without neovim.

git/gitconfig

@@ -11,7 +11,6 @@
 	tool = nvim -d
 	conflictstyle = zdiff3
 [core]
-	editor = nvim
 	eol = native
 	pager = delta || (diff-so-fancy | less --tabs=4 -RFX)
 	quotepath = false