Remove deprecated error prop to Slider

[erikras]

Fixes #114.

[codecov-io]

Codecov Report

Merging #137 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #137   +/-   ##
=======================================
  Coverage   78.37%   78.37%           
=======================================
  Files          11       11           
  Lines          37       37           
=======================================
  Hits           29       29           
  Misses          8        8

Impacted Files	Coverage Δ
src/Slider.js	100% <ø> (ø)	⬆️

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