[PBNTR-431] Fixing textarea duplicated classname (#3660)

**What does this PR do?**
Removing textarea duplicated class name causing duplicated
margin-bottom.

**Screenshots:**

![image](https://github.com/user-attachments/assets/ae07f263-049b-440f-bc0a-ce81d964516c)

**How to test?** 
1. Go to the Textarea doc page


#### Checklist:
- [x] **LABELS** Add a label: `enhancement`, `bug`, `improvement`, `new
kit`, `deprecated`, or `breaking`. See [Changelog &
Labels](https://github.com/powerhome/playbook/wiki/Changelog-&-Labels)
for details.
- [x] **DEPLOY** I have added the `milano` label to show I'm ready for a
review.
- [ ] **TESTS** I have added test coverage to my code.

playbook/app/pb_kits/playbook/pb_textarea/_textarea.tsx

@@ -92,7 +92,6 @@ const Textarea = ({
       <Caption text={label} />
       {children || (
         <textarea
-            className="pb_textarea_kit"
             disabled={disabled}
             name={name}
             onChange={onChange}

playbook/app/pb_kits/playbook/pb_textarea/textarea.html.erb

@@ -15,7 +15,6 @@
     <%= text_area(
         :object,
         :method,
-        :class => "#{object.classname}",
         :max_characters => object.max_characters,
         :name => object.name,
         :onkeyup => object.onkeyup,