chore: Remove mail.send helpers with on-behalf-of header

.gitignore

@@ -6,3 +6,4 @@ coverage.txt
 sendgrid.env
 .vscode
 prism*
+**/.idea/**/*

USAGE.md

@@ -6000,20 +6000,20 @@ if err != nil {
 <a name="on-behalf-of"></a>
 # On-Behalf of Subuser
 
-The on-behalf-of header allows you to make calls for a particular subuser through the parent account; this can be useful for automating bulk updates or administering a subuser without changing authentication in your code.
-## With Mail Helper Class
+The `on-behalf-of` header allows you to make calls for a particular subuser through the parent account. This can be 
+useful for automating bulk updates or administering a subuser without changing authentication in your code.
 
-```go
-
-client := sendgrid.NewSendClientSubuser(os.Getenv("SENDGRID_API_KEY"), "SUBUSER_USERNAME")
-
-```
-
-## Without Mail Helper Class
+## Note
+The v3/mail/send endpoint does not support the `on-behalf-of` header. ([Source](
+    https://docs.sendgrid.com/api-reference/how-to-use-the-sendgrid-v3-api/on-behalf-of-subuser
+))
 
 ```go
 
-request := sendgrid.GetRequestSubuser(os.Getenv("SENDGRID_API_KEY"), "/v3/mail/send", "https://api.sendgrid.com", "SUBUSER_USERNAME")
+request := sendgrid.GetRequestSubuser(
+	os.Getenv("SENDGRID_API_KEY"), "/v3/tracking_settings/subscription", 
+	"https://api.sendgrid.com", "SUBUSER_USERNAME",
+)
 
 ```
 

sendgrid.go

@@ -47,11 +47,3 @@ func NewSendClient(key string) *Client {
 	request.Method = "POST"
 	return &Client{request}
 }
-
-// GetRequestSubuser like NewSendClient but with On-Behalf of Subuser
-// @return [Client]
-func NewSendClientSubuser(key, subuser string) *Client {
-	request := GetRequestSubuser(key, "/v3/mail/send", "", subuser)
-	request.Method = "POST"
-	return &Client{request}
-}

sendgrid_test.go

@@ -84,11 +84,6 @@ func TestGetRequestSubuser(t *testing.T) {
 	ShouldHaveHeaders(&request, t)
 }
 
-func TestNewSendClientSubuser(t *testing.T) {
-	client := NewSendClientSubuser("API_KEY", "subuserUsername")
-	ShouldHaveHeaders(&client.Request, t)
-}
-
 func getRequest(endpoint string) rest.Request {
 	return GetRequest("SENDGRID_APIKEY", endpoint, "")
 }