Remove support for AWS_CREDENTIAL_FILE.

[Kyle Smith]

This used to be required internally, but we removed this requirement in my project at least. AWS SDK no longer uses this file, so we shouldn't encourage deprecated credentials management.

Ping @pdrakeweb @glennpratt @borsothy @cpliakas @itafroma, do any of you rely on this behavior still. If so, any opposition to taking it out in 1.0.0 and updating your projects before upgrading?

[Chris Pliakas]

@askreet Nope, not using it. Thanks for checking!

[Mark Trapp]

Not using it, either!

[Gergely Borsothy-Gaal]

Not that I'm aware of.