Remove universal wheel, python 2 is unsupported

[gopackgo90]

Issue description

This library doesn't seem to support python 2 but is still producing a universal wheel, setup.cfg can just be removed since that is the only entry.

https://github.com/braintree/braintree_python/blob/9b58e54eee72a7682bf341e4c9a8a404199e7c3c/setup.cfg

[hollabaq86]

👋 @gopackgo90 thanks for reaching out, and this is a valid callout and something we missed in the last major version update. Will keep this open until it's resolved.

[hollabaq86]

for internal tracking, ticket 2049