[lldb][NFCI] Remove UserExpression::GetJITModule

UserExpression::GetJITModule was used to support an option in
UserExpression::Evaluate that let you hold onto the JIT Module used during
the expression evaluation. This was only actually used in one spot --
REPL::IOHandlerInputComplete. That method didn't actually take use the
JIT module it got back, so this feature was not used in practice.
This means that we can delete the support in UserExpression::Evaluate
and delete the UserExpression::GetJITModule method entirely.