ref-manual: Removed PRINC entry from the glossary

This variable has been deprecated a while and Paul Eggleton suggested
we finally remove it from the glossary.  I removed it and also
took away the two links from where the variable was referenced
in a couple spots in the migration section.

(From yocto-docs rev: 2b3c762428bd9ca64087ba6359c4eb5fb5a3db84)

Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/migration.xml

@@ -1351,8 +1351,7 @@
             <title><filename>PRINC</filename></title>
 
             <para>
-                The
-                <link linkend='var-PRINC'><filename>PRINC</filename></link>
+                The <filename>PRINC</filename>
                 variable has been deprecated and triggers a warning if
                 detected during a build.
                 For
@@ -2247,8 +2246,7 @@
             The following QA Check and Validation Changes have occurred:
             <itemizedlist>
                 <listitem><para>
-                    Usage of
-                    <link linkend='var-PRINC'><filename>PRINC</filename></link>
+                    Usage of <filename>PRINC</filename>
                     previously triggered a warning.
                     It now triggers an error.
                     You should remove any remaining usage of