Drop SCANBUILD environment variable

It was intended to make ninja pass the `--status-bugs` argument to
the scan-build tool, but it turns out that it never did this properly
and as of meson 0.52.0, the parsing of that variable changed to
require a full path, breaking the CI builds. This patch makes it
call the standard `ninja scan-build` and I'll work with meson
upstream to add a way to invoke `--status-bugs`.

Signed-off-by: Stephen Gallagher <sgallagh@redhat.com>

.travis/scanbuild.sh

@@ -11,7 +11,7 @@
 # For more information on free software, see
 # <https://www.gnu.org/philosophy/free-sw.en.html>.
 
-SCANBUILD="scan-build --status-bugs" ninja scan-build
+ninja scan-build
 if [ $? -eq 0 ]; then
   exit 0
 else