docs: kernel-documentation: remove some highlight directives

With the conf.py change, we don't need them to avoid warnings anymore.
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/kernel-documentation.rst

@@ -366,8 +366,6 @@ Domain`_ references.
 Cross-referencing from reStructuredText
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-.. highlight:: none
-
 To cross-reference the functions and types defined in the kernel-doc comments
 from reStructuredText documents, please use the `Sphinx C Domain`_
 references. For example::
@@ -390,8 +388,6 @@ For further details, please refer to the `Sphinx C Domain`_ documentation.
 Function documentation
 ----------------------
 
-.. highlight:: c
-
 The general format of a function and function-like macro kernel-doc comment is::
 
   /**
@@ -572,8 +568,6 @@ DocBook XML [DEPRECATED]
 Converting DocBook to Sphinx
 ----------------------------
 
-.. highlight:: none
-
 Over time, we expect all of the documents under ``Documentation/DocBook`` to be
 converted to Sphinx and reStructuredText. For most DocBook XML documents, a good
 enough solution is to use the simple ``Documentation/sphinx/tmplcvt`` script,