ref(proj-config): Remove span attributes option

[iker-barriocanal]

#64346 removes the usage of the option, and this PR removes the option itself.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (1153a9e) 81.36% compared to head (089b242) 81.28%.
Report is 3 commits behind head on master.

❗ Current head 089b242 differs from pull request most recent head 1f6af68. Consider uploading reports for the commit 1f6af68 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #64347      +/-   ##
==========================================
- Coverage   81.36%   81.28%   -0.08%     
==========================================
  Files        5237     5236       -1     
  Lines      231110   230473     -637     
  Branches    45342    45230     -112     
==========================================
- Hits       188033   187334     -699     
- Misses      37228    37298      +70     
+ Partials     5849     5841       -8     

Files	Coverage Δ
src/sentry/models/options/project_option.py	97.43% <ø> (-0.04%)	⬇️
src/sentry/projectoptions/defaults.py	100.00% <ø> (ø)
src/sentry/relay/config/__init__.py	92.01% <ø> (-0.06%)	⬇️

... and 832 files with indirect coverage changes