Clean up TrajectoryExecutionManager API: remove unused code

[cambel]

Part of #3156.

The idea is to first clean up the unused code of the API before adding new futures to handle the simultaneous execution of trajectories.

[codecov]

Codecov Report

Merging #3178 (8e37718) into master (79626a6) will increase coverage by 0.24%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3178      +/-   ##
==========================================
+ Coverage   61.71%   61.95%   +0.24%     
==========================================
  Files         375      375              
  Lines       33270    33135     -135     
==========================================
- Hits        20530    20524       -6     
+ Misses      12740    12611     -129     

Impacted Files	Coverage Δ
moveit_ros/planning/moveit_cpp/src/moveit_cpp.cpp	68.51% <ø> (+1.07%)	⬆️
...y_execution_manager/trajectory_execution_manager.h	100.00% <ø> (ø)
...ution_manager/src/trajectory_execution_manager.cpp	60.55% <ø> (+7.45%)	⬆️
...ipulation/pick_place/src/manipulation_pipeline.cpp	71.91% <0.00%> (-1.65%)	⬇️
...on/pick_place/src/approach_and_translate_stage.cpp	75.16% <0.00%> (-0.60%)	⬇️
...dl_kinematics_plugin/src/kdl_kinematics_plugin.cpp	77.91% <0.00%> (-0.37%)	⬇️
...raint_samplers/src/default_constraint_samplers.cpp	81.00% <0.00%> (-0.29%)	⬇️
...meterization/work_space/pose_model_state_space.cpp	81.77% <0.00%> (+1.77%)	⬆️
.../ompl_interface/src/detail/constrained_sampler.cpp	60.98% <0.00%> (+17.08%)	⬆️

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[simonschmeisser]

Please add a note to MIGRATION.md explaining what's going on

[v4hn]

This was approved in yesterday's Maintainer Meeting.