Do not export the SuiteSparse Partition target

The Partition target merely signals that CHOLMOD was compiled with METIS
and adds nothing beyond CHOLMOD and METIS. Exporting it as a dependency
of Ceres forced consumers to repeat the partitioning check in their own
environment. If the check failed there, the Ceres target referred to a
missing target and CMake generation failed.

Link CHOLMOD and METIS directly and use the Partition component only to
enable CHOLMOD partitioning.

Fixes #1172

Change-Id: If9ea17dd462f3712e92039c0c0217545ba2ea57e