remove "incidentally" exported internal function

`*Config.ConfigureTLS()` is invoked internally by `NewClient` and API
consumers should not invoke directly.

Now that http client is created in `api.NewClient`,
`*Config.ConfigureTLS` makes no sense.  API consumers that call it
explicitly can remove the invocation and preserve the behavior.

api/api.go

@@ -319,18 +319,6 @@ func cloneWithTimeout(httpClient *http.Client, t time.Duration) (*http.Client, e
 	return &nc, nil
 }
 
-// ConfigureTLS applies a set of TLS configurations to the the HTTP client.
-//
-// Deprecated: This method is called internally.  Consider using ConfigureTLS instead.
-func (c *Config) ConfigureTLS() error {
-
-	// preserve backward behavior where ConfigureTLS pre0.9 always had a client
-	if c.HttpClient == nil {
-		c.HttpClient = defaultHttpClient()
-	}
-	return ConfigureTLS(c.HttpClient, c.TLSConfig)
-}
-
 // ConfigureTLS applies a set of TLS configurations to the the HTTP client.
 func ConfigureTLS(httpClient *http.Client, tlsConfig *TLSConfig) error {
 	if tlsConfig == nil {