docs(spm-mm): remove reference to SEL2 SPMC

As the SEL2 SPMC design doc is migrated to Hafnium tree, remove the
reference to this implementation from TF-A's SPM-MM doc.

Signed-off-by: Olivier Deprez <olivier.deprez@arm.com>
Change-Id: I1609c7d1d098420412dffc7b1309cc9c11502f8a

docs/components/secure-partition-manager-mm.rst

@@ -4,17 +4,10 @@ Secure Partition Manager (MM)
 Foreword
 ========
 
-Two implementations of a Secure Partition Manager co-exist in the TF-A codebase:
-
--  SPM based on the FF-A specification (:ref:`Secure Partition Manager`).
--  SPM based on the MM interface.
-
-Both implementations differ in their architectures and only one can be selected
-at build time.
-
-This document describes the latter implementation where the Secure Partition Manager
-resides at EL3 and management services run from isolated Secure Partitions at S-EL0.
-The communication protocol is established through the Management Mode (MM) interface.
+This document describes the implementation where the Secure Partition Manager
+resides at EL3 and management services run from isolated Secure Partitions at
+S-EL0. The communication protocol is established through the Management Mode
+(MM) interface.
 
 Background
 ==========