remove nerdtree submodule
Squashed 'bundle/nerdtree/' content from commit b0bb781

git-subtree-dir: bundle/nerdtree
git-subtree-split: b0bb781fc73ef40365e4c996a16f04368d64fc9d
Merge commit 'effe8cf7f7552e7d3dfc5d5e1da753e12ce072fb' as 'bundle/nerdtree'