Remove incorrect spline-conversions

[gilwoolee]

We have noticed that we convert Spline to Interpolated incorrectly or treat Spline as if it's only a set of Waypoints. This PR removes all such use cases:
Removes

• computeKunzTiming(Spline)
• convertToInterpolated(Spline)
• concatenate(Spline, Spline)
• convertToKunzPath(Spline)
• toR1JointTrajectory(Spline)

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[brianhou]

Looks good to me!

[codecov]

Codecov Report

Merging #511 into master will increase coverage by 0.07%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master     #511      +/-   ##
==========================================
+ Coverage   77.67%   77.75%   +0.07%     
==========================================
  Files         262      262              
  Lines        6792     6713      -79     
==========================================
- Hits         5276     5220      -56     
+ Misses       1516     1493      -23

Impacted Files	Coverage Δ
src/trajectory/util.cpp	92.25% <ø> (+4.67%)	⬆️
include/aikido/planner/kunzretimer/KunzRetimer.hpp	100% <ø> (ø)	⬆️
src/planner/kunzretimer/KunzRetimer.cpp	70.93% <0%> (-3.47%)	⬇️
src/planner/parabolic/ParabolicUtil.cpp	97.89% <100%> (+5.3%)	⬆️