doc: Remove wayland-util.c from file list

Documentation generation via doxygen includes wayland-util.c in its file
list. Although functions are documented in wayland-util.h, doxygen is
not automatically using the same documentation for functions in
wayland-util.c. In addition, everything listed in the doxygen page for
wayland-util.c is documented in the page for wayland-util.h and the
pages for corresponding structures. As such, the doxygen page for
wayland-util.c has no value, and is redundant.

Remove the doxygen page for wayland-util.c.

Signed-off-by: Yong Bakos <ybakos@humanoriented.com>
Acked-by: Peter Hutterer <peter.hutterer@who-t.net>