.travis.yml: drop setting default log output

The log backend is the default one, we don't need to explicitly set it.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

.travis.yml

@@ -50,7 +50,7 @@ env:
     - MAKEFLAGS="-j3"
   matrix:
     - CONFIG=""
-    - CONFIG="--enable-debug --enable-debug-tcg --enable-trace-backends=log"
+    - CONFIG="--enable-debug --enable-debug-tcg"
     - CONFIG="--disable-linux-aio --disable-cap-ng --disable-attr --disable-brlapi --disable-uuid --disable-libusb"
     - CONFIG="--enable-modules --disable-linux-user"
     - CONFIG="--with-coroutine=ucontext --disable-linux-user"