Drop unused ResiliencePipelineRegistry APIs

[martintmk]

Details on the issue fix or feature implementation

This makes the registry append-only.

One reason the removals might not be required is the built-in dynamic reload support. Instead of removing, re-creating and adding the pipeline back, it's cleaner and safer to just tell the registry to reload the pipeline.

If there is a feature request for it, we can add these APIs easily back.

Contributes to #1233

Confirm the following

• I started this PR by branching from the head of the default branch
• I have targeted the PR to merge into the default branch
• I have included unit tests for the issue/feature
• I have successfully run a local build

[codecov]

Codecov Report

Merging #1495 (819e4a2) into main (57c540d) will decrease coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1495      +/-   ##
==========================================
- Coverage   83.64%   83.59%   -0.05%     
==========================================
  Files         268      268              
  Lines        6394     6377      -17     
  Branches     1007     1007              
==========================================
- Hits         5348     5331      -17     
  Misses        837      837              
  Partials      209      209              

Flag	Coverage Δ
linux	83.59% <ø> (-0.05%)	⬇️
macos	83.59% <ø> (-0.05%)	⬇️
windows	83.59% <ø> (-0.05%)	⬇️

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Files Changed	Coverage Δ
...ore/Registry/ResiliencePipelineRegistry.TResult.cs	100.00% <ø> (ø)
.../Polly.Core/Registry/ResiliencePipelineRegistry.cs	100.00% <ø> (ø)
...dencyInjection/PollyServiceCollectionExtensions.cs	100.00% <ø> (ø)