cmDyndepCollation: Drop outdated mentions of CXX_MODULE_INTERNAL_PARTITIONS

These were left from an older design iteration in which, for MSVC, we
needed to distinguish `cl -internalPartition` from `cl -interface`
before scanning.  It is no longer needed since `cl -scanDependencies`
was updated to use the standard-conforming interpretation of
non-exported module partition syntax.

Issue: #24611