Remove mentions of DWM_Full, which was removed by #18

Pull request #18 refactored the layout code, but did not re-implement the
DWM_Full function that fullscreened a window. Since the C-M command was still
referenced by the documentation, it was confusing that the fullscreen command
stopped working after updating dwm.vim.
Merge pull request #53 from martinmelin/remove-mentions-of-fullscreen

Remove mentions of DWM_Full, which was removed by #18