Failpoints.Disable not remove failpath from fps.reg

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do? If possible, provide a recipe for reproducing the error.
   I'm trying to iterate over failpoint.List() to obtain all the enabled failpoint keys. However, I'm also retrieving deprecated keys from the list.
2. What did you expect to see?
   After calling failpoint.disable(key), the key should no longer be present in fps.reg
3. What did you see instead?
   Now, even after I disable a failpoint key, I can still retrieve it once it has been enabled.4. Versions of the failpoint

   • failpoint-ctl version (run failpoint-ctl -V):

     (paste failpoint-ctl version here)
     ReleaseVersion None
     BuildTS None
     GitHash None
     GitBranch None
     GoVersion None
     

Start by tracing the failpoint.List() and failpoint.Disable() entry points and inspect how enabled keys are stored in fps.reg. Reproduce the sequence of enabling, disabling, and listing a key, then verify that disabled keys no longer appear in the list.