revert: rewrite of the concatenation operators

[jponge]

Reverts b552fc4, we will eventually need to come back to rewriting this operator.

[jponge]

/cc @ozangunalp if you can check against Reactive Messaging before I proceed to a 2.5.3 release 🙏

[codecov]

Codecov Report

Merging #1467 (87a4eb3) into main (c59be05) will decrease coverage by 0.10%.
Report is 1 commits behind head on main.
The diff coverage is 84.46%.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1467      +/-   ##
============================================
- Coverage     89.32%   89.22%   -0.10%     
  Complexity     3362     3362              
============================================
  Files           459      459              
  Lines         13402    13399       -3     
  Branches       1641     1640       -1     
============================================
- Hits          11971    11955      -16     
- Misses          797      814      +17     
+ Partials        634      630       -4     

Files	Coverage Δ
...llrye/mutiny/operators/multi/MultiConcatMapOp.java	82.40% <84.46%> (-3.54%)	⬇️

... and 12 files with indirect coverage changes

[ozangunalp]

Lgtm