(sql-xemacs-p, sql-emacs19-p, sql-emacs20-p): Remove.

(sql-xemacs-p, sql-emacs19-p, sql-emacs20-p): Remove.
(sql-mode-syntax-table): Use shared GNU EMacs/XEmacs syntax.
(sql-builtin-face, sql-doc-face): Remove.
(sql-mode-ansi-font-lock-keywords)
(sql-mode-oracle-font-lock-keywords)
(sql-mode-postgres-font-lock-keywords)
(sql-mode-linter-font-lock-keywords)
(sql-mode-ms-font-lock-keywords)
(sql-mode-mysql-font-lock-keywords): Use standard fonts.
(sql-product-font-lock): Fix font-lock reset when font rules change.
(sql-highlight-product): Remove incorrect font-lock reset logic.