vimrc: disable preview + <leader>g CtrlPTag + tabs

The preview does not play well with YouCompleteMe plugin and Go. Plus it
isn't required because it does not show docs anyway.

Have not yet spent the time to figure out generating ctags for Go files
per buffer to make effective use of CtrlPBufTag. Plus the global search
tends to be more usable most of the times anyway. (You still can use the
TagBar if you need to.)

Using spaces instead of tabs was good for Python. No longer true for Go.
Also since goimports formats the Go code the shortcut to remove trailing
space has been removed.