[lit] Remove unused TestingProgressDisplay attr

Summary:
`TestingProgressDisplay` initializes its `current` attribute to `None`, but
never reads or writes the value again. Remove it.

Reviewers: echristo, delcypher, beanz, ddunbar

Subscribers: llvm-commits, mehdi_amini

Differential Revision: https://reviews.llvm.org/D25415

llvm-svn: 283709