Restore missing Ogre2Heightmap::DestroyImpl implementation

[iche033]

🦟 Bug fix

Summary

Added missing Ogre2Heightmap::DestroyImpl implementation. The changes were added to ign-rendering6 branch in #670 but looks like they got lost in the 6->7 merge.

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 #870 (93aedfe) into gz-rendering7 (8b47e3f) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 93aedfe differs from pull request most recent head 61d7cc5. Consider uploading reports for the commit 61d7cc5 to get more accurate results

@@              Coverage Diff               @@
##           gz-rendering7     #870   +/-   ##
==============================================
  Coverage          77.22%   77.23%           
==============================================
  Files                170      170           
  Lines              14734    14738    +4     
==============================================
+ Hits               11379    11383    +4     
  Misses              3355     3355           

Files Changed	Coverage Δ
ogre2/src/Ogre2Heightmap.cc	78.71% <100.00%> (+0.43%)	⬆️