Remove mention of removed dhcp_options_enabled

Commit 3292103d80676fc789615465759de966dba4e9f7 removed
dhcp_options_enabled, remove it from the help string as well.

Change-Id: I1f89e0b809c5a47b02a11f2c013a8cbb6d0afa4a
Closes-Bug: #1289993
Merge "Remove mention of removed dhcp_options_enabled"