Don't hard-require Qt5::WebEngineWidgets or Qt5::WebKitWidgets.

Gentoo last-rited dev-qt/qtwebengine:5 (5.15.16_p20241115 was the
final tree version, dropped 2025-04-28) and dev-qt/qtwebkit long
before that. Neither is available in ::gentoo today.

The C++ side never calls QWebEngine* APIs directly -- the web widgets
were linked only so the Python help-browser code in
qt/python/mantidqt/mantidqt/widgets/helpwindow/ could dlopen them via
qtpy. Those imports will fail gracefully at runtime (the fallback
stub in helpwindowview.py already handles the absent-binding case and
limits search).

Replace the detect block with empty stubs so the C++ build proceeds
without a web-widgets link.

--- a/qt/widgets/common/CMakeLists.txt
+++ b/qt/widgets/common/CMakeLists.txt
@@ -651,28 +651,12 @@
   COMPONENTS Concurrent Help Network PrintSupport
   REQUIRED
 )
-# Prefer WebEngineWidgets over WebkitWidgets
-unset(Qt5_FOUND)
-find_package(
-  Qt5 ${QT_MIN_VERSION}
-  COMPONENTS WebEngineWidgets
-  QUIET
-)
-if(Qt5_FOUND)
-  set(_webwidgets_tgt Qt5::WebEngineWidgets)
-else()
-  find_package(
-    Qt5 ${QT_MIN_VERSION}
-    COMPONENTS WebKitWidgets
-    QUIET
-  )
-  if(Qt5_FOUND)
-    set(_webwidgets_tgt Qt5::WebKitWidgets)
-    set(_webengine_def USE_QTWEBKIT)
-  else()
-    message(FATAL_ERROR "Unable to find suitable module for web widgets. Tried: WebEnginewidgets, WebKitWidgets")
-  endif()
-endif()
+# Gentoo: Qt5::WebEngineWidgets and Qt5::WebKitWidgets are unavailable
+# (qtwebengine:5 and qtwebkit last-rited). Python help-browser imports
+# QtWebEngineWidgets via qtpy and handles its absence at runtime; the
+# C++ build does not touch the APIs at all.
+set(_webwidgets_tgt "")
+set(_webengine_def "")

 # If not sip header is defined we need to generate one with sip-module
 if(SIP_INCLUDE_DIR AND EXISTS ${SIP_INCLUDE_DIR}/sip.h)