fix: remove retryDelay option

[JustinBeckwith]

fixes #143

[codecov]

Codecov Report

Merging #203 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #203   +/-   ##
=======================================
  Coverage   96.64%   96.64%           
=======================================
  Files           4        4           
  Lines         597      597           
  Branches      100       99    -1     
=======================================
  Hits          577      577           
  Misses         19       19           
  Partials        1        1

Impacted Files	Coverage Δ
src/index.ts	97.36% <100%> (ø)	⬆️
src/common.ts	100% <100%> (ø)	⬆️
src/retry.ts	98.68% <100%> (-0.01%)	⬇️
src/gaxios.ts	93.46% <100%> (ø)	⬆️

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