Remove the animation of the search icon in SearchField

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.
2. Search boxes in Plasma and KDE apps are generally wide enough, that hiding
   one icon doesn't add much to it.
3. 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