Drop the ResilienceStrategyBuilder.IsGenericBuilder property

[martintmk]

Details on the issue fix or feature implementation

Follow-up of #1430

It was internal API that's not required anymore.

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 #1431 (940a005) into main (2410e68) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1431      +/-   ##
==========================================
- Coverage   83.93%   83.91%   -0.02%     
==========================================
  Files         276      276              
  Lines        6528     6520       -8     
  Branches     1005     1005              
==========================================
- Hits         5479     5471       -8     
  Misses        840      840              
  Partials      209      209              

Flag	Coverage Δ
linux	?
macos	83.91% <100.00%> (-0.02%)	⬇️
windows	83.91% <100.00%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
...rc/Polly.Core/ResilienceStrategyBuilder.TResult.cs	100.00% <ø> (ø)
src/Polly.Core/ResilienceStrategyBuilder.cs	100.00% <ø> (ø)
src/Polly.Core/ResilienceStrategyBuilderBase.cs	100.00% <ø> (ø)
src/Polly.Core/ResilienceStrategyBuilderContext.cs	100.00% <ø> (ø)
src/Polly.Core/Fallback/FallbackHandler.cs	100.00% <100.00%> (ø)
...ack/FallbackResilienceStrategyBuilderExtensions.cs	100.00% <100.00%> (ø)
...ging/HedgingResilienceStrategyBuilderExtensions.cs	100.00% <100.00%> (ø)