Remove duplicate methods of value_and_hessian

[adrhill]

Remove duplicate methods of value_and_hessian and value_gradient_and_hessian for HigherOrderBackends.

These two methods are duplicates of the implementations for AbstractBackends.
Since HigherOrderBackend is an AbstractBackend, they are not required.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (211b675) 82.94% compared to head (9c81c3c) 82.72%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #126      +/-   ##
==========================================
- Coverage   82.94%   82.72%   -0.22%     
==========================================
  Files           8        8              
  Lines         428      411      -17     
==========================================
- Hits          355      340      -15     
+ Misses         73       71       -2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.