[7.0] Remove banner helper

[laoneo]

• I read the Generative AI policy and my contribution is either not created with the help of AI or is compatible with the policy and GNU/GPL 2 or later.

Summary of Changes

Remove banner helper class as it is deprecated and not used anymore.

Testing Instructions

Code review as it is not used.

Link to documentations

Please select:

• Documentation link for guide.joomla.org:

• No documentation changes for guide.joomla.org needed

• Pull Request link for manual.joomla.org: Remove banner helper Manual#663

• No documentation changes for manual.joomla.org needed

[QuyTon]

I have tested this item ✅ successfully on 4b03853

---

_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/47937.}

[joomdonation]

I have tested this item ✅ successfully on 4b03853

---

_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/47937.}

[joomdonation]

RTC

---

_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/47937.}