fix: Default hit cdn for dcs (#244)

Before:
```
url is https://statsigapi.net/v1/download_config_specs/secret-B3bwGac5BMDKJY5IBvJYHHyworp4RECb8uOUBRl7uB8.json?sinceTime=0

[Go] 
url is https://statsigapi.net/v1/get_id_lists

[Go] 
url is https://statsigapi.net/v1/log_event
```
Test:
Logged the api
```
url is  https://api.statsigcdn.com/v1/download_config_specs/secret-B3bwGac5BMDKJY5IBvJYHHyworp4RECb8uOUBRl7uB8.json?sinceTime=0

[Go] 
url is  https://statsigapi.net/v1/get_id_lists

[Go] 
url is  https://statsigapi.net/v1/log_event
```

Author: xinlili-statsig

client.go

@@ -45,9 +45,6 @@ func newClientImpl(sdkKey string, options *Options) (*Client, *initContext) {
 	context := newInitContext()
 	diagnostics := newDiagnostics(options)
 	diagnostics.initialize().overall().start().mark()
-	if len(options.API) == 0 {
-		options.API = "https://statsigapi.net/v1"
-	}
 	errorBoundary := newErrorBoundary(sdkKey, options, diagnostics)
 	if !options.LocalMode && !strings.HasPrefix(sdkKey, "secret") {
 		err := errors.New(InvalidSDKKeyError)