[Frontend] Remove unused FileMgr in pp arg parse

Summary:
A FIXME added 8 years ago (2010) in https://reviews.llvm.org/rL118203
mentioned that a FileManager should not need to be used when parsing
preprocessor arguments. In fact, its only use was removed 6 years ago
(2012), in https://reviews.llvm.org/rL166452. Remove the unused
variable and the obsolete FIXME.

Test Plan: `check-clang`

Reviewers: v.g.vassilev

Reviewed By: v.g.vassilev

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D41867

llvm-svn: 322118