cleanup for warnings by staticcheck

the warnings that remained were either unused code that i wanted to use in the
future, or other type's of todo's. i've been mentally ignoring them, assuming i
would get back to them soon enough to fix them. but that hasn't happened yet,
and it's better to have a clean list with only actual isses.

integration_test.go

@@ -3,7 +3,6 @@
 package main
 
 import (
-	"context"
 	"crypto/tls"
 	"fmt"
 	"net"
@@ -21,8 +20,6 @@ import (
 	"github.com/mjl-/mox/smtpclient"
 )
 
-var ctxbg = context.Background()
-
 func tcheck(t *testing.T, err error, errmsg string) {
 	if err != nil {
 		t.Helper()