drop support for non-CLIENT_INTERFACE builds

Maintaining non-CLIENT_INTERFACE builds is a minor pain point and the define complicates the code base. We have put all the performance-affecting pieces of the client interface into runtime options, such that a CLIENT_INTERFACE build without a client should be as performant as a non-CLIENT_INTERFACE build. There seems to be little reason to keep the define around given that these days there are no major users (that we know of) of other configs.