cmake: Fix VERSION warning in doc build

The 'doc' build has been invoking 'project' with the argument "VERSION
${PROJECT_VERSION}". This is cargo-cult code inherited from the
toplevel build system.

This can be safely removed because in the doc build the versioning
variables that are set by 'project' are unused and PROJECT_VERSION is
not available to be de-referenced at that point in time any way ...

This fixes #12282

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>