Removed unnecessary code to handle start point in lineSliceAlong()

[jush]

Removed code which could never occur when calculating the initial point in lineSliceAlong().

Given that travelled > startDist then double overshot = startDist - travelled could never be 0.

Also added a test to validate when travelled == stopDist.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.35%. Comparing base (26a10e0) to head (a62f863).
⚠️ Report is 1 commits behind head on main.

@@             Coverage Diff              @@
##               main    #1631      +/-   ##
============================================
+ Coverage     79.23%   79.35%   +0.11%     
- Complexity     1128     1129       +1     
============================================
  Files           162      162              
  Lines          4701     4698       -3     
  Branches        660      659       -1     
============================================
+ Hits           3725     3728       +3     
+ Misses          706      702       -4     
+ Partials        270      268       -2     

Files with missing lines	Coverage Δ
...s-turf/src/main/java/com/mapbox/turf/TurfMisc.java	88.69% <ø> (+3.31%)	⬆️

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