docs: Remove "could not extract kernel version" warning

This warning will happen for every normal kernel docs build and doesn't
carry any useful information.  Should anybody actually depend on this
"version" variable (which isn't clear to me), the "unknown version" value
will be clue enough.
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/conf.py

@@ -88,7 +88,6 @@ finally:
     if makefile_version and makefile_patchlevel:
         version = release = makefile_version + '.' + makefile_patchlevel
     else:
-        sys.stderr.write('Warning: Could not extract kernel version\n')
         version = release = "unknown version"
 
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