Remove deprecated functions and classes

[azeey]

Summary

This removes all deprecated functions and classes:

• sdf::Element::SetInclude
• sdf::Element::GetInclude
• sdf::Color
• sdf::JointAxis::InitialPosition
• sdf::JointAxis::SetInitialPosition

Test it

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge

[codecov-commenter]

Codecov Report

Merging #622 (efe5ff1) into main (2d298f0) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

❗ Current head efe5ff1 differs from pull request most recent head c9c0d86. Consider uploading reports for the commit c9c0d86 to get more accurate results

@@            Coverage Diff             @@
##             main     #622      +/-   ##
==========================================
- Coverage   87.59%   87.57%   -0.02%     
==========================================
  Files          72       72              
  Lines       10513    10497      -16     
==========================================
- Hits         9209     9193      -16     
  Misses       1304     1304              

Impacted Files	Coverage Δ
include/sdf/Element.hh	100.00% <ø> (ø)
include/sdf/Types.hh	100.00% <ø> (ø)
src/JointAxis.cc	96.85% <ø> (-0.17%)	⬇️
src/Element.cc	97.64% <100.00%> (-0.05%)	⬇️

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[scpeters]

the ign-physics nightly build is broken since we haven't merged gazebosim/gz-physics#276 yet