remove harbor-postgres-primary PodDisruptionBudget

This MR improves the PDB for harbor-postgres to not set a PDB for postgres-primary.

Indeed setting a PDB is useless because this StatefulSet never has more than 1 replica (this chart does not have an HA design) and setting a PBD with default settings may sometimes prevents node drains (see #1560 (closed) and #1575 (closed))