powerpc/pseries: remove memory "re-add" implementation

dlpar_memory() no longer has any callers which pass
PSERIES_HP_ELOG_ACTION_READD. Remove this case and the corresponding
unreachable code.

Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200612051238.1007764-17-nathanl@linux.ibm.com