Remove twoCharacterId

[zackcl]

Describe the issue
We've decided to get rid of the twoCharacterId property from the experiment data structure as they are not being used.

So a decision point (partition) and a condition shouldn't have the twoCharacterId field.

linked issue: #1244

[amurphy-cl]

The Java client library is expecting this, so we'd need to update the current and lower versions of the client lib

[amurphy-cl]

MATHia is using v3 of the client library, so we will need to get MATHia on v5 before removing this.

[amurphy-cl]

MATHia is now on v5 of the library, check to see if it's expecting the twoCharacterId