This removes indirect dependencies from go.mod, and
includes the fix for the missing -help flag info.

The latest cobra release is v1.2.1. Given that there were
minor security-related dependency cleanup during v1.2 release,
I was tempted to pick up the latest version, but that caused
dependency updates in golang.org/x/sys and golang.org/x/tools
which may be too recent (golang.org/x/* follow the go's release
support policy, so recent versions may not be compatible with
go versions beyond go's official version support policy).

Verified that dlv still builds with go1.12.x.
(go1.12 is the oldest version of go that can build the latest delve already).

$ go get -d github.com/spf13/cobra@v1.1.3
$ go mod tidy
$ go mod vendor
$ go run _scripts/gen-usage-docs.go