Fix memory leaks from dart::gui::osg::Viewer (#1331)

[jslee02]

This PR fixes #1331.

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[codecov]

Codecov Report

Merging #1349 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1349      +/-   ##
==========================================
+ Coverage   56.87%   56.88%   +<.01%     
==========================================
  Files         366      366              
  Lines       27174    27174              
==========================================
+ Hits        15456    15458       +2     
+ Misses      11718    11716       -2

Impacted Files	Coverage Δ
dart/dynamics/Skeleton.cpp	65.95% <0%> (-0.17%)	⬇️
dart/dynamics/EulerJoint.cpp	70.73% <0%> (+3.04%)	⬆️