[ABNF] Remove 'string' from the keywords.

Now that the stdlib includes the type alias declaration

type string = [char; _];

in order for that type declaration to be legal and not "special", 'string' must
be an identifier.

grammar/abnf-grammar.txt

@@ -414,7 +414,6 @@ keyword = %s"address"
         / %s"Self"
         / %s"self"
         / %s"static"
-        / %s"string"
         / %s"type"
         / %s"u8"
         / %s"u16"