[Maps][Alerting] Remove selection of time-field in track-config from geo containment

[thomasneirynck]

Geo-threshold jobs should not allow for the selection of the time-field in the tracks-index. This should always correspond to the default time-field of the index-pattern.

The reason for this is to remove any ambiguity with other applications within Kibana:

• in Discover, which cannot show documents based on different time-field
• in Maps, which cannot show documents based on different time-field

The consequence is that if a users selects a time-field which is not the time-field of the index-pattern, the configuration is essentially unusable for further visualization and exploration in other Kibana applications.

The ability to select a time-field is actually an edge-case, and essentially duplicates a config-option the user has already made when creating the index-pattern.

cc @kmartastic @aaronjcaldwell

[elasticmachine]

Pinging @elastic/kibana-gis (Team:Geo)

[elasticmachine]

Pinging @elastic/kibana-alerting-services (Team:Alerting Services)

[kmartastic]

Good catch @thomasneirynck

[elasticmachine]

Pinging @elastic/kibana-presentation (Team:Presentation)

[nreese]

Solution should not effect previously created rules. The solution should just hide time field selection in UI to prevent new rules from being created with time field.