tests/docker: remove unnecessary default definitions

The definition of DOCKER_IMAGES and DOCKER_TESTS copes already with an
empty value of $(IMAGES) and $(TESTS), no need to force them to "%" if
undefined.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20220401141326.1244422-5-pbonzini@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220419091020.3008144-8-alex.bennee@linaro.org>

tests/docker/Makefile.include

@@ -8,11 +8,6 @@ COMMA := ,
 
 HOST_ARCH = $(if $(ARCH),$(ARCH),$(shell uname -m))
 
-# These variables can be set by the user to limit the set of docker
-# images and tests to a more restricted subset
-TESTS ?= %
-IMAGES ?= %
-
 DOCKER_FILES_DIR := $(SRC_PATH)/tests/docker/dockerfiles
 # we don't run tests on intermediate images (used as base by another image)
 DOCKER_PARTIAL_IMAGES := debian10 debian11