FindHDF5: Drop IMPORTED_IMPLIB property from imported UNKNOWN libraries

The imported targets added by commit a8e0a6b3e4 (FindHDF5: Port changes
from VTK, 2020-06-10, v3.19.0-rc1~312^2~1) to use config-specific
locations set the `IMPORTED_IMPLIB` target property.  That property has
no meaning for an imported library with type `UNKNOWN`.  Drop it to
avoid confusion.