tests/docker: remove SID_AGE test hack

Now we are using "named" snapshots of debian-sid we can rely on the
existing checksum mechanism for detecting changes.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>

tests/docker/Makefile.include

@@ -98,19 +98,6 @@ docker-image-debian-s390x-cross: docker-image-debian9
 docker-image-debian-win32-cross: docker-image-debian8-mxe
 docker-image-debian-win64-cross: docker-image-debian8-mxe
 
-# Debian SID images - we are tracking a rolling distro so we want to
-# force a re-build of the base image if we ever need to build one of
-# its children.
-ifndef SKIP_DOCKER_BUILD
-ifeq ($(HAVE_USER_DOCKER),y)
-SID_AGE=$(shell $(DOCKER_SCRIPT) check --checktype=age --olderthan=180 --quiet qemu:debian-sid)
-ifeq ($(SID_AGE),)
-else
-docker-image-debian-sid: NOCACHE=1
-endif
-endif
-endif
-
 docker-image-debian-alpha-cross: docker-image-debian-sid
 docker-image-debian-hppa-cross: docker-image-debian-sid
 docker-image-debian-m68k-cross: docker-image-debian-sid