Remove deprecated APIs before 2.0.0 release

[JoshRosen]

Before releasing 2.0.0, we should audit the APIs deprecated in #65 and remove them if nobody has asked us to keep them.

[JoshRosen]

Originally I planned this for 0.6.0 but I think I'll bump to 0.7.0, where we might choose to drop support for really old Spark versions.