[cmake] Remove the LLDB versions of the exception-controlling variables

Summary:
One can still use the LLVM variables to control this: LLVM_ENABLE_EH, LLVM_ENABLE_RTTI. It's not
clear to me why one would want to control these at lldb level and it's generally not even a good
idea to compile parts of the same binary with different values of these flags.

Reviewers: zturner, tfiala

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D20673

llvm-svn: 270863