stake: Remove feature stake_merge_with_unmatched_credits_observed

[joncinque]

Problem

While going through the stake program code, I noticed that stake_merge_with_unmatched_credits_observed is still in there, when it's been enabled for over two years. And then I confirmed that it was mine in #18985

Summary of Changes

Remove the feature gate, and update a test that was an old function. The removed test case is no longer valid since we relaxed the credits_observed equality requirement.

Fixes #

[codecov]

Codecov Report

Merging #34114 (fe28b06) into master (f349d71) will increase coverage by 0.0%.
Report is 4 commits behind head on master.
The diff coverage is 100.0%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #34114   +/-   ##
=======================================
  Coverage    81.9%    81.9%           
=======================================
  Files         816      816           
  Lines      219717   219701   -16     
=======================================
- Hits       179964   179959    -5     
+ Misses      39753    39742   -11