fix(otlp-exporter-base): avoid adding new userAgent option for browser exporters

[pichlermarc]

Which problem is this PR solving?

Follow-up to #5928, as I missed it while doing the review. I noticed this while working on #5994.

The userAgent option is not used in any of the browser transports. This PR removes it from the OtlpHttpConfiguration interface so that it's only present on OtlpNodeHttpConfiguration and OtlpGrpcConfiguration where it's actually used. 🙂

Skipping changelog since the change from #5928 is unreleased.

cc @david-luna

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.23%. Comparing base (fe18523) to head (637aeea).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6013   +/-   ##
=======================================
  Coverage   95.23%   95.23%           
=======================================
  Files         311      311           
  Lines        8597     8597           
  Branches     1801     1801           
=======================================
  Hits         8187     8187           
  Misses        410      410           

Files with missing lines	Coverage Δ
...-base/src/configuration/otlp-http-configuration.ts	100.00% <ø> (ø)
.../src/configuration/otlp-node-http-configuration.ts	100.00% <ø> (ø)

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[david-luna]

You're right, is not needed for now. But IIUC we may need to review this if we want to remove the legacy options :) I got a bit lost navigating the interfaces 😅