client.NewClientWithOpts(): remove redundant type assertion (gosimple)

As caught by gosimple:

    client/client.go:138:14: S1040: type assertion to the same type: c.client.Transport already has type http.RoundTripper (gosimple)
        if _, ok := c.client.Transport.(http.RoundTripper); !ok {
                    ^

This check was originally added in dc9f5c2ca3cdf8fef5786a80a0a1b0e7c18d4420, to
check if the passed option was a `http.Transport`, and later changed in
e345cd12f96b3985257652b606c8463a5772944b to check for `http.RoundTripper` instead.

Client.client is a http.Client, for which the Transport field is a RoundTripper,
so this check is redundant.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

client/client.go

@@ -135,9 +135,6 @@ func NewClientWithOpts(ops ...Opt) (*Client, error) {
 		}
 	}
 
-	if _, ok := c.client.Transport.(http.RoundTripper); !ok {
-		return nil, fmt.Errorf("unable to verify TLS configuration, invalid transport %v", c.client.Transport)
-	}
 	if c.scheme == "" {
 		c.scheme = "http"