SCHED-1897: Remove alloc_mem_used health check

[fabrizio2210]

Summary

• remove the obsolete built-in alloc_mem_used drain and undrain checks
• remove their default Helm registrations
• keep [user_problem] alloc_mem_used as the replacement check's stable drain-reason identifier so nodes drained before the upgrade can still be recovered

Why

This PR is related to SCHED-1897.

The periodic idle_mem_used check introduced by #2773 replaces the old prolog-time alloc_mem_used behavior for detecting suspicious memory use. Removing the old check avoids running two overlapping memory checks with different execution models.

Dependency

This PR is stacked on #2773 and deliberately targets agent/sched-1897-idle-memory-check instead of main. Merge #2773 first, then retarget this PR to main.

This ordering ensures that alloc_mem_used is not removed before its idle_mem_used replacement is available.

For safe upgrades with persisted Helm values, merge the independent, main-based #2779 before this PR. It adds generic validation for enabled built-ins whose bundled files no longer exist.

Upgrade impact

• Existing nodes whose drain reason starts with [user_problem] alloc_mem_used remain recoverable because the replacement drain and undrain schedules preserve that stable reason prefix.
• CHECKS_JOB_ALLOC_MEM_BYTES support remains available to custom checks.

Persisted-values warning

Deployments that explicitly retain slurmScripts.builtIn.alloc_mem_used.* with enabled: true must remove those entries during the upgrade. Without #2779, Helm can retain the removed built-in keys, render missing script/configuration content, and produce a malformed checks configuration that prevents the check runner from running health checks.

#2779 changes this failure mode into a clear Helm rendering error. It is kept separate so this PR contains only the check removal and its direct drain-reason compatibility.

Validation

• python3 -m unittest discover -s helm/slurm-cluster/slurm_scripts -p '*_test.py' — 21 tests passed
• helm unittest helm/slurm-cluster — 101 tests passed across 18 suites
• default helm template rendering passed
• YAML and JSON parsing passed
• git diff --check passed