Drop support for xopen v1

[victorlin]

Description of proposed changes

Following through with deprecation to simplify the code.

Related issue(s)

Follow-up to deprecation in #1532

Checklist

• Automated checks pass
• Check if you need to add a changelog message
• Check if you need to add tests
• Check if you need to update docs
• Post-merge: release this month as part of version 27.0.0

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.31%. Comparing base (037525d) to head (cc6d19a).
Report is 11 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1692   +/-   ##
=======================================
  Coverage   72.30%   72.31%           
=======================================
  Files          79       79           
  Lines        8276     8271    -5     
  Branches     1691     1691           
=======================================
- Hits         5984     5981    -3     
+ Misses       2007     2005    -2     
  Partials      285      285           

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