docker: Clean the MXE base image

Using the duplicated same package is confusing.
Reported-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>

tests/docker/dockerfiles/debian8-mxe.docker

@@ -14,6 +14,6 @@ RUN apt-get update
 RUN DEBIAN_FRONTEND=noninteractive eatmydata \
     apt-get install -y --no-install-recommends \
         libpython2.7-stdlib \
-        $(apt-get -s install -y --no-install-recommends gw32.shared-mingw-w64 gw32.shared-mingw-w64 | egrep "^Inst mxe-x86-64-unknown-" | cut -d\  -f2)
+        $(apt-get -s install -y --no-install-recommends gw32.shared-mingw-w64 | egrep "^Inst mxe-x86-64-unknown-" | cut -d\  -f2)
 
 ENV PATH $PATH:/usr/lib/mxe/usr/bin/