test/iscsi_tgt: remove nightly iSCSI + pmem test

This test ran for 10 minutes, which is half of the allotted time for
the entire nightly test run.

Additionally, we should be testing each bdev backend at the component
level (e.g. via bdevio and bdevperf), rather than trying to add tests
for the whole matrix of front-end app (iscsi_tgt, nvmf_tgt, etc.) with
each bdev backend.

Change-Id: I10311e3e5092084dc939ab8662e8312066ecab6e
Signed-off-by: Daniel Verkamp <daniel.verkamp@intel.com>
Reviewed-on: https://review.gerrithub.io/391303


Tested-by: SPDK Automated Test System <sys_sgsw@intel.com>
Reviewed-by: Changpeng Liu <changpeng.liu@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>