aws s3 commands should not be checking regions

[jamesls]

There's currently code in the high level s3 commands that validate that a region is specified (either through a CLI param, env vars, or config file). However, botocore does not require a region as s3 is marked as a "global" service at the botocore layer. This makes the region validation inconsistent between s3 and s3api, as s3api does not require a region.

We should be able to safely remove check_region in this code.