[opt-viewer] Remove positional arg checks (NFC)

Summary:
opt-stats.py and opt-viewer.py's argument parsers both take a positional
argument 'yaml_files'. Positional arguments in Python's argparse module are
required by default, so the subsequent checks for `len(args.yaml_files) == 0`
are unnecessary -- if the length was zero, then the call to
`parser.parse_args()` would have thrown an error already.

Because there is no way for `len(args.yaml_files)` to be zero at these
points, removing the code is NFC.

Reviewers: anemet, davidxl

Reviewed By: anemet

Subscribers: llvm-commits, fhahn

Differential Revision: https://reviews.llvm.org/D34567

llvm-svn: 306147