stream: simplify nested PromisePrototypeThens

[aduh95]

No need to complicate them

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.25%. Comparing base (a1e3e6d) to head (e2bc65f).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #64470   +/-   ##
=======================================
  Coverage   90.24%   90.25%           
=======================================
  Files         741      741           
  Lines      241604   241588   -16     
  Branches    45526    45536   +10     
=======================================
+ Hits       218029   218036    +7     
+ Misses      15107    15101    -6     
+ Partials     8468     8451   -17     

Files with missing lines	Coverage Δ
lib/internal/webstreams/adapters.js	86.42% <100.00%> (-0.20%)	⬇️

... and 28 files with indirect coverage changes

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