Travis: don't build same configuration multiple times

In ArduPlane "make all" is the same as "make sitl" and both sitl and
linux are already covered by the loop below.  Don't build the same
configuration multiple times.

Tools/scripts/build_all_travis.sh

@@ -12,7 +12,7 @@ set -x
 
 echo "Testing ArduPlane build"
 pushd ArduPlane
-for b in all apm2 sitl linux; do
+for b in apm2; do
     pwd
     make clean
     make $b -j4