[analyzer] Drop explicit mention of range constraint solver

Summary: The basic constraint solver was dropped in rL162384, leaving the range constraint solver as the default and only constraint solver. Explicitly specifying it is unnecessary, and makes it difficult to test with other solver backends.

Reviewers: zaks.anna, dcoughlin

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D26694

llvm-svn: 288372