consul: fix formatting of consul.agent_join

[multani]

What does this PR do?

This patch removes the wrong copy/paste and the usage of the undefined enable argument.

What issues does this PR fix or reference?

None

Previous Behavior

Using the consul.agent_join function raises a KeyError: 'enable' message, cf. the code in https://github.com/saltstack/salt/blob/develop/salt/modules/consul.py#L612

• there's no reference to enable anywhere, and it is not documented anywhere
• actually, the message formatted which causes this error is completely unrelated to the "join" function of Consul, and looks to be a copy/paste from the agent_maintenance function

Tests written?

No

[rallytime]

Thanks @multani