delete_variable() should return boolean

### 🔍 Before submitting the issue

- [x] I have searched among the existing issues
- [x] I am using a Python virtual environment

### 🐞 Description of the bug

This method should return a boolean.

### 📝 Steps to reproduce

NA

### 💻 Which operating system are you using?

Windows

### 📀 Which ANSYS version are you using?

NA

### 🐍 Which Python version are you using?

3.9

### 📦 Installed packages

```shell
NA
```

Locate the delete_variable() entry point and inspect any existing tests around it. Verify the current return behavior, add coverage for the expected boolean result, and run the relevant test suite to confirm the method now returns a boolean.

Written by the indexing model from the issue text.