Add XmpMeta::delete_struct_field

[scouten-adobe]

Checklist

• This PR represents a single feature, fix, or change.
• All applicable changes have been documented.
• Any TO DO items (or similar) have been entered as GitHub issues and the link to that issue has been included in a comment.

[codecov]

Codecov Report

Merging #131 (8bf047b) into main (d3509f4) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #131      +/-   ##
==========================================
+ Coverage   97.34%   97.38%   +0.04%     
==========================================
  Files           7        7              
  Lines        1204     1225      +21     
==========================================
+ Hits         1172     1193      +21     
  Misses         32       32              

Impacted Files	Coverage Δ
src/ffi.rs	100.00% <ø> (ø)
src/xmp_meta.rs	99.76% <100.00%> (+<0.01%)	⬆️

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