Remove spf-check debugging tool

The spf-check debugging tool is quite primitive, rarely (if ever) used,
not tested, and has been superceded by the one in the spf library
(https://blitiri.com.ar/git/r/spf/b/master/t/cmd/spf-check/f=spf-check.go.html).

This patch removes it.

cmd/spf-check/spf-check.go

@@ -1,24 +0,0 @@
-// Command line tool for playing with the SPF library.
-//
-// Not for use in production, just development and experimentation.
-//go:build !coverage
-// +build !coverage
-
-package main
-
-import (
-	"flag"
-	"fmt"
-	"net"
-
-	"blitiri.com.ar/go/spf"
-)
-
-func main() {
-	flag.Parse()
-
-	r, err := spf.CheckHostWithSender(
-		net.ParseIP(flag.Arg(0)), "", flag.Arg(1))
-	fmt.Println(r)
-	fmt.Println(err)
-}