test: drop redundant IMAGE_NAME=

If it is not specified, then "default" will be used.

test/TEST-03-JOBS/test.sh

@@ -4,7 +4,6 @@ set -e
 
 TEST_DESCRIPTION="Job-related tests"
 TEST_NO_QEMU=1
-IMAGE_NAME="default"
 
 # shellcheck source=test/test-functions
 . "${TEST_BASE_DIR:?}/test-functions"

test/TEST-17-UDEV/test.sh

@@ -3,7 +3,6 @@
 set -e
 
 TEST_DESCRIPTION="UDEV"
-IMAGE_NAME="default"
 TEST_NO_NSPAWN=1
 
 # shellcheck source=test/test-functions

test/TEST-64-UDEV-STORAGE/test.sh

@@ -9,7 +9,6 @@
 set -e
 
 TEST_DESCRIPTION="systemd-udev storage tests"
-IMAGE_NAME="default"
 TEST_NO_NSPAWN=1
 # Save only journals of failing test cases by default (to conserve space)
 TEST_SAVE_JOURNAL="${TEST_SAVE_JOURNAL:-fail}"