docs: remove plat_convert_pk() interface from release doc

The code was already removed as part of commit 4ac5b3949d87
"refactor(auth): replace plat_convert_pk". The present commit just
removes it from the release documentation.

Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>
Change-Id: I06b35f110c844267d69a865df55dd451ed2f08cd
diff --git a/docs/about/release-information.rst b/docs/about/release-information.rst
index c0a344e..0ba0a7a 100644
--- a/docs/about/release-information.rst
+++ b/docs/about/release-information.rst
@@ -67,7 +67,7 @@
 |                                | Date        | after   |                                                         |
 |                                |             | Release |                                                         |
 +================================+=============+=========+=========================================================+
-| plat_convert_pk() function     |   Nov'22    |   2.9   | Platform conversion to manage specific PK hash          |
+| None at this time              |             |         |                                                         |
 +--------------------------------+-------------+---------+---------------------------------------------------------+
 
 Removal of Deprecated Drivers