Use default physics engine in example worlds

[iche033]

🦟 Bug fix

Related issue: gazebosim/gazebo_test_cases#339

Summary

When launching a world with TPE and the world that has nested models, the following errors flood the console:

[Err] [Physics.cc:2889] Internal error: a physics entity ptr with an ID of [9] does not exist.
[Err] [Physics.cc:2889] Internal error: a physics entity ptr with an ID of [7] does not exist.

I think nested models are be supported in dart now so updated the examples worlds to use the default physics engine

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 #2134 (0c3b846) into gz-sim7 (9055d88) will increase coverage by 0.02%.
The diff coverage is n/a.

❗ Current head 0c3b846 differs from pull request most recent head 35da510. Consider uploading reports for the commit 35da510 to get more accurate results

@@             Coverage Diff             @@
##           gz-sim7    #2134      +/-   ##
===========================================
+ Coverage    64.99%   65.01%   +0.02%     
===========================================
  Files          356      356              
  Lines        28929    28929              
===========================================
+ Hits         18801    18807       +6     
+ Misses       10128    10122       -6     

see 1 file with indirect coverage changes