fix(otlp-exporter-base): remove xhr transport

[cjihrig]

Which problem is this PR solving?

The XhrTransport is deprecated and pending removal.

Fixes #6315

Short description of the changes

This commit implements the changes described in #6315.

Type of change

• Breaking change (fix or feature that would cause existing functionality to not work as expected)

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 95.46%. Comparing base (adbaa90) to head (5f3a0eb).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6317   +/-   ##
=======================================
  Coverage   95.46%   95.46%           
=======================================
  Files         317      317           
  Lines        9599     9599           
  Branches     2218     2218           
=======================================
  Hits         9164     9164           
  Misses        435      435           

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[pichlermarc]

Looks good, thanks!

[otelbot]

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