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
1 file changed