Minor ResiliencePipelineRegistry cleanup of internals

[martintmk]

Details on the issue fix or feature implementation

Dropping some ifdefs in favor of introducing legacy bridge.

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 #1505 (259b337) into main (3f3e6f0) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1505   +/-   ##
=======================================
  Coverage   83.90%   83.91%           
=======================================
  Files         273      273           
  Lines        6499     6503    +4     
  Branches     1026     1026           
=======================================
+ Hits         5453     5457    +4     
  Misses        837      837           
  Partials      209      209           

Flag	Coverage Δ
linux	?
macos	83.91% <100.00%> (+<0.01%)	⬆️
windows	83.91% <100.00%> (+<0.01%)	⬆️

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