[lit] Remove dead code (not referenced anywhere), and clarify some function names.

Summary:
The dead code seems to be unreferenced, according to textual search across the
LLVM SVN repo.

The clarification part of this change alters the name of a module-level function
so that it is different from the name of the class-methods that call it.
Currently, there are no erroneous references, but stylistically (c.f. PEP-8),
internal "helper" functions should generally be named accordingly by prepending
an underscore. (I also chose to add '_impl', which isn't necessary, but helps me
at least to mentally disambiguate the interface and implementation functions.)

Reviewers: zturner, modocache

Subscribers: sanjoy, llvm-commits

Differential Revision: https://reviews.llvm.org/D34775

llvm-svn: 306600