package/libva-utils: drop _SOURCE variable which has the default value

package/libva-utils: drop _SOURCE variable which has the default value

Since commit b0907949261ef5e8179c027c086903f30417dd66
("package/libva-utils: bump to version 2.3.0"), the LIBVA_UTILS_SOURCE
variable has the default value of the <pkg>_SOURCE variable, so
check-package complains:

package/libva-utils/libva-utils.mk:8: remove default value of _SOURCE variable (http://nightly.buildroot.org/#generic-package-reference)

Let's fix this by dropping the now unneeded variable assignment.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>