Drop qtwebengine from qtdoc's dependencies

qtwebengine is no longer part of qt5.git, but qtdoc still lists it as an
optional dependency, so Coin builds it on every configuration qtdoc
integrates on. When qtwebengine fails to build on a configuration,
qtdoc's integrations fail as well, even for documentation-only changes,
as happened when the HarmonyOS x86_64 configuration was added. The
dependency also holds qtdoc's dependency updates back until
qtwebengine's go through.

Only the documentviewer demo's PDF plugin and the photosurface demo's
intro slides use it, through Qt PDF. qtdoc's CMake already treats Qt PDF
as optional, so CI no longer builds the PDF plugin, but it still builds
anywhere Qt PDF is installed. Photosurface only needs Qt PDF at runtime.

Fixes: QTBUG-151373
Change-Id: I8a477b344dbb6e18f0b3571af56afa70d0fea361
Reviewed-by: Topi Reinio <topi.reinio@qt.io>
Reviewed-by: Moss Heim <moss.heim@qt.io>