feat(nvim-gtk): Update color reset steps for colorscheme onedark

In order to be able to re-load/reset default settings in colorscheme
onedark, there is no longer a function (`onedark#extend_highlight`) to
re-apply colors, but instead that can be achieved by re-loading the
highlighting with the `syntax reset` command.

nvim/ginit.vim

@@ -13,6 +13,5 @@ endif
 " Restore theme's background on GUI
 let g:onedark_config = {'transparent': v:false}
 if exists('g:colors_name') && g:colors_name == 'onedark'
-	let s:colors = onedark#GetColors()
-	call onedark#extend_highlight('Normal', {'bg': {'gui': s:colors.black.gui}})
+	syntax reset
 endif