docs: Do not seek comments in kernel/rcu/tree_plugin.h

There are no kerneldoc comments in this file, so do not attempt to
include them in the docs build.
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/core-api/kernel-api.rst

@@ -358,8 +358,6 @@ Read-Copy Update (RCU)
 
 .. kernel-doc:: kernel/rcu/tree.c
 
-.. kernel-doc:: kernel/rcu/tree_plugin.h
-
 .. kernel-doc:: kernel/rcu/tree_exp.h
 
 .. kernel-doc:: kernel/rcu/update.c

Documentation/driver-api/basics.rst

@@ -115,9 +115,6 @@ Kernel utility functions
 .. kernel-doc:: kernel/rcu/tree.c
    :export:
 
-.. kernel-doc:: kernel/rcu/tree_plugin.h
-   :export:
-
 .. kernel-doc:: kernel/rcu/update.c
    :export: