graphics/kphotoalbum: Remove WEBENGINE option

With the addition of the MARBLE option in 687ac26b32a7, we can be a bit
more clever. Various Qt components are only linked to if the MARBLE
option is enabled and the WEBENGINE option in astro/marble is enabled.
This avoids potentially conflicting settings of the WEBENGINE option in
astro/marble and the former WEBENGINE option in this port.

Disable Marble detection the cannonical way to avoid CMake warnings.