[cmake] Remove LLDB_ENABLE_PYTHON_SCRIPTS_SWIG_API_GENERATION.

Summary:
This should be a mandatory build process going forward, if Python
is enabled. The longer term desire is to remove the old shell
scripts entirely.

Reviewers: zturner, clayborg, labath

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D12667

llvm-svn: 246979