Remove the legacy search stack and its config flags

[pringelmann]

Legacy is the accessibility fallback, so it goes only once the new search has an independent a11y signoff. In practice that means graduating unified search out from behind the enable_non-accessible_features switch. Decide the two legacy-only behaviours first: enableLiveSearch (type-then-Enter) and the in:/- inline filter syntax.

• BITV / WCAG 2.1 AA sign-off obtained for the new search
• enableLiveSearch + inline-filter-syntax carry-or-drop decision recorded
• legacy view files + bundle + webpack entry removed
• unified_search.enabled / enable_non-accessible_features gate removed

[susnux]

I think we should already remove the legacy search, the old one is not actively monitored for accessibility but the new one is regularly checked.