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.

Modules/FindHDF5.cmake

@@ -1067,7 +1067,6 @@ if (HDF5_FOUND)
         endif ()
         set_target_properties("hdf5::${hdf5_target_name}" PROPERTIES
           IMPORTED_LOCATION "${_hdf5_location}"
-          IMPORTED_IMPLIB "${_hdf5_location}"
           INTERFACE_INCLUDE_DIRECTORIES "${HDF5_${hdf5_lang}_INCLUDE_DIRS}"
           INTERFACE_COMPILE_DEFINITIONS "${_hdf5_definitions}")
         if (_hdf5_libtype STREQUAL "SHARED")
@@ -1127,7 +1126,6 @@ if (HDF5_FOUND)
         string(REPLACE "-D" "" _hdf5_definitions "${HDF5_${hdf5_lang}_HL_DEFINITIONS}")
         set_target_properties("hdf5::${hdf5_target_name}" PROPERTIES
           IMPORTED_LOCATION "${_hdf5_location}"
-          IMPORTED_IMPLIB "${_hdf5_location}"
           INTERFACE_INCLUDE_DIRECTORIES "${HDF5_${hdf5_lang}_HL_INCLUDE_DIRS}"
           INTERFACE_COMPILE_DEFINITIONS "${_hdf5_definitions}")
         if (_hdf5_libtype STREQUAL "SHARED")