Remove the option to specify the language name as a single-quoted
string.  This has been obsolete since ee8ed85d.  Removing it allows
better grammar refactoring.

The syntax of the CREATE FUNCTION LANGUAGE clause is not changed.

Discussion: https://www.postgresql.org/message-id/flat/163c00a5-f634-ca52-fc7c-0e53deda8735%402ndquadrant.com