Remove alterations exclusion and enrollment tables on experiment exclusion changes

[bcb37]

For this issue functionality was added to remove users and groups from the enrollment tables when exclusions were added to experiments. This was never fully implemented, since subsegments were not considered - either in the case where an exclusion of type "segment" is added, or in the case where such an excluded subsegment is itself edited. After reevaluation, it was decided to remove this functionality.

Specified new behavior:

• Once a student or group is in the enrollment table, they can not be removed by editing segments or experiments
• Similarly, the exclusions table should not be affected by changes made to segments or experiments
• /mark-ing after edits to experiment exclusions have been made should work as it currently does (i.e. by adding entries to the exclusions table)
• The data displayed in the UI and included in exports will contain all enrollments. The only effect of adding experiment exclusions will be to change the behavior of future API calls.