Remove make_keywords

It is not used anymore.

src/tools/make_keywords

@@ -1,15 +0,0 @@
-#!/bin/sh
-
-# src/tools/make_keywords
-
-cat <<END
-To get a list of keywords compared to SQL'92, take the keywords out of
-backend/parser/keywords.c and tools/SQL_keywords.
-
-Then run sdif with the PostgreSQL keyword file first, then the SQL'92
-keywords.  Here is what I used:
-
-sdif /tmp/pgkeywords tools/SQL_keywords |\
-sed 's/</ /' | sed 's/>/ /'|sed 's/|/\
-                                        /' | sort -b +0
-END