Revert "TEST-55-OOMD: Verify that ExecStopPost= runs on oom-kill"

The test consistently fails on CentOS 9:

TEST-55-OOMD.sh[678]: + test -f /run/testbloat-exec-stop-post
[FAILED] Failed to start TEST-55-OOMD.service.

https://github.com/systemd/systemd/actions/runs/17689186773/job/50293446228?pr=38911

This reverts commit 5bf7438ff0.

test/integration-tests/TEST-55-OOMD/TEST-55-OOMD.units/TEST-55-OOMD-testbloat.service

@@ -8,4 +8,3 @@ Description=Create a lot of memory pressure
 MemoryHigh=3M
 Slice=TEST-55-OOMD-workload.slice
 ExecStart=stress-ng --timeout 3m --vm 10 --vm-bytes 200M --vm-keep
-ExecStopPost=-touch /run/testbloat-exec-stop-post

test/units/TEST-55-OOMD.sh

@@ -133,9 +133,6 @@ test_basic() {
 
 testcase_basic_system() {
     test_basic /TEST.slice/TEST-55.slice/TEST-55-OOMD.slice/TEST-55-OOMD-workload.slice
-
-    # Ensure ExecStopPost= runs on oom-kill.
-    test -f /run/testbloat-exec-stop-post
 }
 
 testcase_basic_user() {