test(otlp-transformer): remove obsolete compat code from toBase64()

[cjihrig]

Which problem is this PR solving?

The toBase64() test helper function contains compatibility code so that it can work in both Node.js and the browser. However, btoa() has been in Node.js since v16.0.0, which went EOL several years ago, so the Node code is dead code.

Fixes # N/A

Short description of the changes

This commit removes the obsolete and dead code from toBase64().

Type of change

Test refactor

How Has This Been Tested?

• Test suite

Checklist:

• Followed the style guidelines of this project
• Unit tests have been added
• Documentation has been updated

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.39%. Comparing base (93d578a) to head (04411ce).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6242   +/-   ##
=======================================
  Coverage   95.39%   95.39%           
=======================================
  Files         314      314           
  Lines        9513     9513           
  Branches     2194     2194           
=======================================
  Hits         9075     9075           
  Misses        438      438           

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