Remove unused var 'errTLSConfigUnavailable' (#29626)

Signed-off-by: yuexiao-wang <wang.yuexiao@zte.com.cn>

client/transport.go

@@ -2,12 +2,9 @@ package client
 
 import (
 	"crypto/tls"
-	"errors"
 	"net/http"
 )
 
-var errTLSConfigUnavailable = errors.New("TLSConfig unavailable")
-
 // transportFunc allows us to inject a mock transport for testing. We define it
 // here so we can detect the tlsconfig and return nil for only this type.
 type transportFunc func(*http.Request) (*http.Response, error)