Delete remaining references to Grammar/Grammar from docs (python#21624)

(Ironically, the file itself remains, see we-like-parsers#135.)

Author: gvanrossum

Doc/library/language.rst

@@ -13,10 +13,8 @@ These modules include:
 
 .. toctree::
 
-   parser.rst
    ast.rst
    symtable.rst
-   symbol.rst
    token.rst
    keyword.rst
    tokenize.rst