docs: Remove mentions of deprecated utils

The utilities reference page contained (unlinked) mentions of long
since deprecated utilities, and since they aren't shipping it's time
to remove them:

	gpsizecalc - removed in June 2010
	gpskew - removed in October 2009
	gprebuildsystem - removed in January 2010
	gpchecknet - removed in April 2010
	gpcheckos - removed in July 2016

Reviewed-by: Lisa Owen
Reviewed-by: David Yozie

gpdb-doc/dita/utility_guide/admin_utilities/util_ref.xml

@@ -34,8 +34,6 @@
           <p>
             <xref href="gpcheckcat.xml#topic1"/>
           </p>
-          <p>gpchecknet (deprecated)</p>
-          <p>gpcheckos (deprecated)</p>
           <p>
             <xref href="gpcheckperf.xml#topic1" type="topic" format="dita"/>
           </p>
@@ -101,7 +99,6 @@
           <p>
             <xref href="gppkg.xml#topic1" type="topic" format="dita"/>
           </p>
-          <p>gprebuildsystem (deprecated)</p>
           <p>
             <xref href="gprecoverseg.xml#topic1" type="topic" format="dita"/>
           </p>
@@ -110,11 +107,9 @@
           </p>
           <p>
             <xref href="gprestore.xml" type="topic" format="dita"/></p>
-          <p>gpsizecalc (deprecated)</p>
           <p>
             <xref href="gpscp.xml#topic1" type="topic" format="dita"/>
           </p>
-          <p>gpskew (deprecated)</p>
           <p>
             <xref href="gpseginstall.xml#topic1" type="topic" format="dita"/>
           </p>