pgsql: Update release checklist to reflect that HISTORY and INSTALL

Log Message:
-----------
Update release checklist to reflect that HISTORY and INSTALL don't
need to be created by hand anymore.

Modified Files:
--------------
pgsql/src/tools:
RELEASE_CHANGES (r1.57 -> r1.58)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/tools/RELEASE_CHANGES.diff?r1=1.57&r2=1.58)