Remove XModuleMixin legacy attibs from Poll and Discussion Block

[salman2013]

Description:

Remove XModuleMixin legacy attributes from Poll and Discussion Block.

Ticket:
#207
#211

Parent Ticket:
#125

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[farhan]

course_key property will be removed

[salman2013]

This property is used at multiple places, I have already updated 'self.scope_ids.usage_id' to 'self.usage_key' inside its returning value.

[farhan]

As per story we should change its usage to context_key

.course_key -> .context_key

[farhan]

Good work
LGTM!