Chore/remove activity content serialization

[wilwade]

Problem

Activity content serialization was breaking with spec if someone used sdk.core.activityContent.serialize link to Pivotal Tracker #179276374

Replaces #144

Solution

Remove methods related to sdk.core.activityContent.serialize

Double Checks:

• [x] Did you update the changelog?
• [x] Any new modules need to be exported?
• [x] Are new methods in the right module?
• [x] Are new methods/modules in core or not (i.e. porcelain or plumbing)?
• [x] Do you have good documentation on exported methods?