Remove CircleCI

CircleCI tests currently seem fail consistently. GitHub actions
introduced in #136 are providing the same coverage (and more), have been
running stable for over a year and are easier to maintain. Thus, remove
Circle CI integration.

Ref. #136 (comment)

Author: tklauser

.circleci/config.yml

@@ -1,7 +1,6 @@
 # gops
 
 [![GitHub Action Status](https://github.com/google/gops/workflows/Tests/badge.svg)](https://github.com/google/gops/actions?query=workflow%3ATests)
-[![Build status](https://circleci.com/gh/google/gops/tree/master.svg?style=shield&circle-token=2637dc1e57d5407ae250480a86a2e553a7d20482)](https://circleci.com/gh/google/gops)
 [![GoDoc](https://godoc.org/github.com/google/gops?status.svg)](https://godoc.org/github.com/google/gops)
 
 gops is a command to list and diagnose Go processes currently running on your system.