Remove forward-ported restriction on model loading

[mjcarroll]

This shouldn't have been ported to gz-sim7: #2103 (comment)

[mjcarroll]

Some new warnings about not using gz::physics::getEngineInstallDir(), but I think that is out of scope for here.

[codecov]

Codecov Report

Merging #2104 (4dbfdeb) into gz-sim7 (f83456f) will decrease coverage by 0.02%.
The diff coverage is n/a.

❗ Current head 4dbfdeb differs from pull request most recent head 12b84d6. Consider uploading reports for the commit 12b84d6 to get more accurate results

@@             Coverage Diff             @@
##           gz-sim7    #2104      +/-   ##
===========================================
- Coverage    64.99%   64.97%   -0.02%     
===========================================
  Files          356      356              
  Lines        28933    28929       -4     
===========================================
- Hits         18805    18798       -7     
- Misses       10128    10131       +3     

Files Changed	Coverage Δ
src/Server.cc	86.25% <ø> (+1.49%)	⬆️

... and 1 file with indirect coverage changes