streams: remove unnecessary check

[aduh95]

+size coerces the a number, so there's no case where its typeof wouldn't be a number.

Also, keep a reference to the original size to return in the RangeError for better UX.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.65%. Comparing base (2a601eb) to head (cb5d3a5).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #63030      +/-   ##
==========================================
- Coverage   89.65%   89.65%   -0.01%     
==========================================
  Files         708      708              
  Lines      220374   220373       -1     
  Branches    42264    42266       +2     
==========================================
- Hits       197584   197576       -8     
- Misses      14656    14670      +14     
+ Partials     8134     8127       -7     

Files with missing lines	Coverage Δ
lib/internal/webstreams/util.js	99.55% <100.00%> (-0.01%)	⬇️

... and 46 files with indirect coverage changes

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