ovsdb: Remove 'managers' column from 'Open vSwitch' table.

ovsdb: Remove 'managers' column from 'Open vSwitch' table.

We had retained but deprecated the use of the older 'managers' column in the
'Open vSwitch' table for compatibility with applications that might still use
it, but that created more problems than it solved. This commit removes the
'managers' column from the schema, and removes all references to it from the
code, init scripts, documentation, and tests.