Sponsorship should be non-obligated for sponsee

[yinyiqian1]

A Sponsorship object lives in both the sponsor's (Owner) and the
sponsee's owner directories, but AccountDelete treated it as a deletion
blocker for both. It should be an obligation only for the sponsor, who
holds its reserve; for the sponsee it must be a non-obligation.
Otherwise anyone could block a sponsee's account deletion simply by
creating a Sponsorship.

related issue: #6892

High Level Overview of Change

Context of Change

API Impact

• Public API: New feature (new methods and/or new fields)
• Public API: Breaking change (in general, breaking changes should only impact the next api_version)
• libxrpl change (any change that may affect libxrpl or dependents of libxrpl)
• Peer protocol change (must be backward compatible or bump the peer protocol version)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.3%. Comparing base (64159eb) to head (9b81f9c).
⚠️ Report is 38 commits behind head on xrplf/sponsor.

Additional details and impacted files

@@               Coverage Diff               @@
##           xrplf/sponsor   #7552     +/-   ##
===============================================
- Coverage           82.6%   82.3%   -0.4%     
===============================================
  Files               1020    1018      -2     
  Lines              78018   78275    +257     
  Branches            7357    8972   +1615     
===============================================
- Hits               64456   64389     -67     
- Misses             13562   13877    +315     
- Partials               0       9      +9     

Files with missing lines	Coverage Δ
...clude/xrpl/tx/transactors/sponsor/SponsorshipSet.h	100.0% <ø> (ø)
.../libxrpl/tx/transactors/Sponsor/SponsorshipSet.cpp	98.0% <100.0%> (-1.5%)	⬇️
...c/libxrpl/tx/transactors/account/AccountDelete.cpp	94.9% <100.0%> (-1.2%)	⬇️

... and 277 files with indirect coverage changes

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[PeterChen13579]

LGTM