The golang.org/x/lint library is marked as deprecated / unmaintained / readonly

This library takes an explicit dependency on it in https://github.com/uber-go/goleak/blob/master/tools_test.go in order to record a specific version of it in go.mod, for use with go install in the make lint target.

The same result could be achieved by installing the same version explicitly in make lint with go install golang.org/x/lint/golint@v0.0.0-20190930215403-16217165b5de, and the tools_test.go and go.mod reference could be dropped, to avoid propagating an unmaintained dependency into downstreams that use this library.