.travis.yml: the xcode10 image seems to be hosed

It fails to install homebrew.  Unfortunately we cannot mark
it as an expected failure because Travis does not match
allow_failures rows against include rows (only against the
main test matrix, which we do not use at all), so just disable
it.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20190220105131.23479-1-pbonzini@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>

.travis.yml

@@ -174,13 +174,6 @@ matrix:
       compiler: clang
 
 
-    - env:
-        - CONFIG="--target-list=i386-softmmu,ppc-softmmu,ppc64-softmmu,m68k-softmmu,x86_64-softmmu"
-      os: osx
-      osx_image: xcode10
-      compiler: clang
-
-
     # Python builds
     - env:
         - CONFIG="--target-list=x86_64-softmmu"