Remove the animation of the search icon in SearchField

(please keep the discussion in https://invent.kde.org/frameworks/plasma-framework/-/merge_requests/829)

Always show the search icon regardless if the search box is focused or not.

Rationale:

1. The current design (hide the search icon when focused) is unusual. Web browsers (e.g. Firefox, Chrome, Edge) and websites (e.g. Google, Gitlab) normally always show the search icon in the search box, so the user probably is expecting that.
1. Search boxes in Plasma and KDE apps are generally wide enough, that hiding one icon doesn't add much to it.
1. The animation is triggered during a tray icon (e.g. NetworkManager) poping up or an app (e.g. Discover) launching. The user doesn't interact with the search box at that time, so the animation can be confusing/distracting. And according to dev (https://bugs.kde.org/show_bug.cgi?id=472391) this can be hard to fix.

BUG: 472391