[analyzer] Delete SMTContext. NFC.

Summary: There is no reason to have a base class for a context anymore as each SMT object carries a reference to the specific solver context.

Reviewers: NoQ, george.karpenkov, hiraditya

Reviewed By: hiraditya

Subscribers: hiraditya, xazax.hun, szepet, a.sidorin, Szelethus

Differential Revision: https://reviews.llvm.org/D50768

llvm-svn: 340532