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 78.20%. Comparing base (5eddaf6) to head (6c24e12).

@@             Coverage Diff              @@
##               main    #1631      +/-   ##
============================================
+ Coverage     78.08%   78.20%   +0.11%     
- Complexity     1070     1071       +1     
============================================
  Files           161      161              
  Lines          4508     4505       -3     
  Branches        626      625       -1     
============================================
+ Hits           3520     3523       +3     
+ Misses          719      715       -4     
+ Partials        269      267       -2     

Files with missing lines	Coverage Δ
...s-turf/src/main/java/com/mapbox/turf/TurfMisc.java	87.80% <100.00%> (+3.37%)	⬆️

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