Don't pollute global scope with atob and btoa

[LiraNuna]

These functions aren't use, and pollute the global scope with unrelated variables.
If this is used as dependency somehow, I highly suggest using a separate package to fill the gap.

Fixes #99

[microshine]

@LiraNuna Thank you for PR
You are right. It's odd code

webcrypto-core has own global btoa and atob functions
https://github.com/PeculiarVentures/webcrypto-core/blob/master/src/base64url.ts#L2-L6

btoa and atob are needed for Base64Url coder https://github.com/PeculiarVentures/webcrypto-core/blob/master/src/base64url.ts#L12

[coveralls]

Coverage decreased (-0.03%) to 90.333% when pulling aca4686 on LiraNuna:remove_atob_btoa into 19d482e on PeculiarVentures:master.