gitlab: Don't define $MAKE

Since libvirt-ci commit 27cfddee8835, paths to build tools such as
ninja and make are exported in the container's environment and can
be used directly.
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>

.gitlab-ci.yml

 variables:
-  MAKE: make
   GIT_DEPTH: 100
 
 stages: