docs: No structured comments in target_core_device.c

Documentation/driver-api/target.rst is seeking kerneldoc comments in
drivers/target/target_core_device.c, but no such comments exist.  Take out
the kernel-doc directive and eliminate one warning from the build.
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/driver-api/target.rst

@@ -10,8 +10,8 @@ TBD
 Target core device interfaces
 =============================
 
-.. kernel-doc:: drivers/target/target_core_device.c
-    :export:
+This section is blank because no kerneldoc comments have been added to
+drivers/target/target_core_device.c.
 
 Target core transport interfaces
 ================================