Remove deprecated sstate-cache-mirror reference

This commit removes 1 of 2 reference, with the other being included in
PR #712.

QA Steps: Checked rendered output, no issues seen.

No related ticket, small fix.

Signed-off-by: Katrina Prosise <katrina.prosise@foundries.io>

source/reference-manual/factory/factory-definition.rst

@@ -206,10 +206,6 @@ Variables
 * **DISTRO**:
                Defines the distro being used.
                Reference: :ref:`ref-linux-distro`.
-* **SSTATE_CACHE_MIRROR**:
-               Defaults to the directory mounted on the SDK build container.
-               If this directory exists, it is used as the source for the shared state cache (``sstate-cache``) mirror.
-               When the directory does not exist, the ``lmp-manifest`` value is used (currently points to the public HTTP shared state cache).
 
 .. _def-containers: