.travis.yml: disable Sparse testing

On travis-ci.org, all builds fail with
   /usr/include/features.h:324:11: error: unable to open bits/predefs.h

With "make docker-travis@ubuntu", they fail with
   /usr/include/features.h:374:13: error: unable to open sys/cdefs.h

With "make docker-travis@fedora", finally, they fail due to sparse
not being able to parse some #pragmas in glib headers.  Just kill
the thing from the CI builds.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
[AJB: tweak title for my OCD]
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>

.travis.yml

@@ -64,9 +64,6 @@ script:
   - make -j3 && ${TEST_CMD}
 matrix:
   include:
-    # Sparse is GCC only
-    - env: CONFIG="--enable-sparse"
-      compiler: gcc
     # gprof/gcov are GCC features
     - env: CONFIG="--enable-gprof --enable-gcov --disable-pie"
       compiler: gcc