ci(custom-checks): remove NodeJS v12 exclusion logic

[petermetz]

While coming up with the fix for #1809
I realized that after upgrading ts-node to the latest available version,
the custom-checks script now can run just fine on NodeJS v12 as well.

Based on the above, we can remove the logic both from tools/ci.sh and
tools/custom-checks/run-custom-checks.ts which stops the custom checks
execution if NodeJS v12 is detected.

The other reason to clean up this from the code is because NodeJS v12 is end of
life now and we no longer include it in the CI at all anyway. So this is one
of those tasks that adds value by removing code instead of adding it.