Remove unnecessary headers to fix ABI checker

[azeey]

🦟 Bug fix

Summary

#2019 added the files
include/ignition/gazebo/ign/gazebo/Export.hh and include/ignition/gazebo/ignition/gazebo/Export.hh which link to <gz/sim/ign/Export.hh>, but that header doesn't exist. This is breaking the ABI checker because it checks every header.

I don't think these headers are needed especially since the ign component is not really public API.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• 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 and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov]

Codecov Report

Merging #2070 (f1621f6) into gz-sim7 (0aadc81) will decrease coverage by 0.05%.
The diff coverage is n/a.

❗ Current head f1621f6 differs from pull request most recent head c39a255. Consider uploading reports for the commit c39a255 to get more accurate results

@@             Coverage Diff             @@
##           gz-sim7    #2070      +/-   ##
===========================================
- Coverage    65.03%   64.99%   -0.05%     
===========================================
  Files          354      354              
  Lines        28688    28688              
===========================================
- Hits         18657    18645      -12     
- Misses       10031    10043      +12     

see 2 files with indirect coverage changes

[azeey]

My guess is, the abi checker is failing to build the stable branch, so it won't pass until we merge this. I'll go ahead and do that and test on another PR.