Don't pass --fast-parser flag anymore

Since January 2017, it is now enabled by default and the only parser available.
python/mypy#2734

Author: flebel

autoload/mypy.vim

@@ -11,5 +11,5 @@
 
 function mypy#ExecuteMyPy()
   silent !clear
-  execute "!mypy --ignore-missing-imports --follow-imports=skip --fast-parser " . bufname("%")
+  execute "!mypy --ignore-missing-imports --follow-imports=skip " . bufname("%")
 endfunction