Remove WithMessageHandler<IHandlerAsync<T>>

[adammorr]

As mentioned in #484 and #471, this PR removes WithMessageHandler<IHandlerAsync<T>> as it hasn't been ported over to the new API and is not something we intend to do

[codecov]

Codecov Report

Merging #624 into master will increase coverage by 0.18%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #624      +/-   ##
==========================================
+ Coverage   77.13%   77.31%   +0.18%     
==========================================
  Files          99       99              
  Lines        2519     2495      -24     
==========================================
- Hits         1943     1929      -14     
+ Misses        576      566      -10

Flag	Coverage Δ
#linux	77.31% <ø> (+0.18%)	⬆️
#osx	43.8% <ø> (+0.65%)	⬆️
#windows	47.33% <ø> (+0.74%)	⬆️

Impacted Files	Coverage Δ
src/JustSaying/JustSayingFluentlyExtensions.cs	81.81% <ø> (+21.81%)	⬆️
src/JustSaying/JustSayingFluently.cs	89.61% <ø> (+0.17%)	⬆️

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