[lit] Remove duplicate to_string method

There are two versions of to_string used by TestRunner.py. The one defined 
in TestRunner.py and the one defined in utils/lit/lit/util.py. The util.py
version is superior to the TestRunner.py version.

This change removes the duplicate to_string in TestRunner.py in favor of
always using the version from util.py. Beside removing duplicate code, this
makes it easier to debug TestRunner.py since only one version of to_string
is used.

Patch by Stella Stamenova!

llvm-svn: 329972