xhtml_style: fix invisible cursor in textarea with chrome

With chrome ( 85.0.4183.108 but I think it appears a few weeks/months ago )
when using erp5_xhtml_style when the cursor in the first column on the left
it's not visible, because the bold background hides it.

Default user agent spreadsheet is a 2px padding, but we have a rule applying a
padding 0 for all textarea. Revert this rule for textarea used as fields in
content.


---

## before:

The cursor is in *Description* field, left to "This style is ..." but cannot be seen.

![image](/uploads/f40262af196d799291643604b91ae2c4/image.png)


## after:

The cursor is same location (*Description* field, left to "This style is ...") and this time is visible.

![image](/uploads/bf1ec63532e2eca1fca7655a4581935a/image.png)

See merge request !1280