Revert ".vimrc: map NERDTree command to Ctrl-N"

This reverts commit 506c0ff683af03431710dd5cfdf4fd7230c55685.

Conflicts:
	.vimrc

Revert ".vim/bundle/nerdtree: add nerdtree plugin submodule"

This reverts commit 57bab55d6465620c400298622ce4b84df6c3349e.

Conflicts:
	.gitmodules