Remove deprecated / obsolete code from multi-domain

[fthobe]

@ikraamg discovered during the review of the restoration PR #172 of this gem several deprecated / obsolete code.

[fthobe]

@kennyadsl @ikraamg

How do you two feel about a major release removing lots of old code?

[rmvirut]

@kennyadsl unrelated but I'm new to ruby on rails but would like to contribute to this extension and the solidus project