[lldb][CPlusPlus][NFCI] Remove redundant construction of ClangASTImporter

The usage of this variable was removed in `4f14c17d`.

This is no longer used inside this file. Since the call to
`GetPersistentExpressionStateForLanguage` has side-effects I marked this
NFCI. But there is no good reason we need this here.

Differential Revision: https://reviews.llvm.org/D157992