[FIX] web: fix form view dropdown's buttons shifted on Edge

In form views, there are stat buttons.

When the window size is too small, some buttons are packed into
a "More" dropdown.

Before this commit:

    - The form view dropdown's buttons are shifted on Microsoft Edge
      and IE 11.

After this commit:

    - The form view dropdown's buttons are not shifted anymore on
      Microsoft Edge and IE 11.

There was a fixed line-height on the button's icon. On Edge and IE 11,
it makes the icon moving down and this causes the offsetting.

The purpose of the line-height was to fix the button height to 44 pixels

So, I set up the button's height and centered the content.

Also, I removed a useless border-left because there was already one
border on the dropdown container.

OPW-2079694

closes odoo/odoo#39380

Signed-off-by: Nans Lefebvre (len) <len@odoo.com>