Update README

Remove the bintray reference from README

Author: thagikura

README.md

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 # FlexboxLayout
 [ ![Circle CI](https://circleci.com/gh/google/flexbox-layout.svg?style=shield&circle-token=2a42716dfffab73d73c5ce7ed7b3ee620cfa137b) ](https://circleci.com/gh/google/flexbox-layout/tree/main)
-[ ![Download](https://api.bintray.com/packages/google/flexbox-layout/flexbox/images/download.svg) ](https://bintray.com/google/flexbox-layout/flexbox/_latestVersion)
 
 FlexboxLayout is a library project which brings the similar capabilities of
 [CSS Flexible Box Layout Module](https://www.w3.org/TR/css-flexbox-1) to Android.