[swig] Remove check_lldb_swig_executable_file_exists.
Code that tried to find swig and then split the path into
a separate path and filename is being removed. The invoking
build system always provides the location of swig and we
don't need to split it into 2 pieces only to recombine it
a short time later.
Reviewers: zturner, domipheus
Differential Revision: http://reviews.llvm.org/D14415