[lit] Remove Python 2.6 and below exec workaround

Summary:
The minimum version of Python required to run LLVM's test suite is 2.7.
Remove a workaround for older Python versions.

Reviewers: echristo, delcypher, beanz, ddunbar

Subscribers: llvm-commits, mehdi_amini

Differential Revision: https://reviews.llvm.org/D25400

llvm-svn: 283705