fix: avoid extra quotes on windows

This reverses the fix from #1256.

refer: #2139, #1256

Author: lervag

autoload/vimtex/process.vim

@@ -167,11 +167,7 @@ if has('win32')
       let l:cmd = 'start /b ' . cmd
     endif
 
-    if self.silent && self.output ==# 'null'
-      let self.prepared_cmd = '"' . l:cmd . '"'
-    else
-      let self.prepared_cmd = l:cmd
-    endif
+    let self.prepared_cmd = l:cmd
   endfunction
 
   " }}}1