.travis.yml: drop xcode9.4 from build matrix

It's broken so it's no longer helping. The latest Xcode is covered by
Cirrus.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20191127132430.3681-1-alex.bennee@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

.travis.yml

@@ -237,13 +237,7 @@ matrix:
         - TEST_CMD=""
 
 
-    # MacOSX builds
-    - env:
-        - CONFIG="--target-list=${MAIN_SOFTMMU_TARGETS}"
-      os: osx
-      osx_image: xcode9.4
-      compiler: clang
-
+    # MacOSX builds - cirrus.yml also tests some MacOS builds including latest Xcode
 
     - env:
         - CONFIG="--target-list=i386-softmmu,ppc-softmmu,ppc64-softmmu,m68k-softmmu,x86_64-softmmu"