Don't escape search text in search page title

Summary:
Doing so makes the title display look ugly and generated a bug report. Instead of escaping
HTML entities, this patch just removes them.

BUG: 416781
FIXED-IN: 5.18.0

Test Plan:
Searching still works

Search terms with HTML-unsafe characters in them no longer look ugly in the title:

{F8070324, size=full}

Reviewers: #vdg, #discover_software_store, apol

Reviewed By: #discover_software_store, apol

Subscribers: plasma-devel, apol

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D27079