Use default tab width in markdown

Also check if +syntax is available before enabling spell.

after/ftplugin/markdown.vim

@@ -1,5 +1,5 @@
 " Enable spelling except in Windows terminal vim.
-if !has("win32") || has("gui_running")
+if has('syntax') && (!has('win32') || has('gui_running'))
   setlocal spell
 endif
 
@@ -31,8 +31,6 @@ endfunction
 
 " Max 80 chars wide.
 setlocal textwidth=80
-" Use tabwidth of 4 to be markdown complient.
-setlocal tabstop=4 shiftwidth=4 softtabstop=4
 
 " Custom surround for markdown link syntax.
 " "ys{motion}l" makes link out of "word" -> "[word]()"