webadmin: removed storage from pools general sub-tab

Since a pool can contain disks from multiple storage domains,
'Resides on Storage Domain' is no longer relevant.
Hence,
* removed from pools general sub-tab.
* removed redundant method PoolGeneralModel -> updateStorageDomain.

Change-Id: I26a1ec85ef404954767aa81042679e3120efad2a
Signed-off-by: Daniel Erez <derez@redhat.com>