switch to vim-go; jnwhiteh/vim-golang is no more

vim-go seems to be pretty nice. it's kind of a super-plugin and replaces a few things, not just syntax. the autofmt that it provides out of the box is also smarter about undo handling.

i'm not 100% sure if it's a drop-in replacement for the four plugins I removed (just that it mentions them in its readme), so you may want to smoke-test the keybindings/integration that you expect from them.

removed the auto fmt command which vim-go handles as well.