[lit] Remove workaround for Python 2.5

Summary:
The minimum version of Python necessary to run the LLVM test suite is
2.7. Code to work around Python 2.5 and lower isn't necessary.

Reviewers: ddunbar, echristo, delcypher, beanz

Subscribers: llvm-commits, mehdi_amini

Differential Revision: https://reviews.llvm.org/D25209

llvm-svn: 283169