Remove [compute-feature-enabled] block_migrate_cinder_iscsi

This option was removed a few years ago[1], and is no longer needed
after queens EOL, according to the commit message.

[1] 48fe99faf2ee92ca1c1e6eeac5519075095f33bc

Change-Id: Ia3985fcdda570c5e0cd32bdd87abfabb0ff516ac
3 files changed