Expression: cleanup unused include

The standard JIT has been discarded in favour of MCJIT.  USE_STANDARD_JIT is no
longer defined.  Furthermore, the execution engine is now built in
IRExecutionUnit.  Simply remove inclusion of both JIT headers.

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@204112 91177308-0d34-0410-b5e6-96231b3b80d8