docs: Do not include kerneldoc comments from kernel/sys.c

...because there are none there, and I cannot figure out what would ever
have been of interest there.  This eliminates this warning:

    ./kernel/sys.c:1: warning: no structured comments found

from the build.
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/driver-api/basics.rst

@@ -103,9 +103,6 @@ Kernel utility functions
 .. kernel-doc:: kernel/panic.c
    :export:
 
-.. kernel-doc:: kernel/sys.c
-   :export:
-
 .. kernel-doc:: kernel/rcu/tree.c
    :export: