Remove compressed point support from EC2 keys

[OR13]

What is the areas you would like to add the new feature to?

Go-COSE Library

Is your feature request related to a problem?

Point compression causes warnings.

What solution do you propose?

Doing point compression in this library is unnecessary, we can require keys to be in uncompressed form, and then throw errors when compressed point keys are passed, and instruct the user to do point compression before calling our interfaces.

What alternatives have you considered?

Doing point compression in our library

Any additional context?

related to #178

and #168 (comment)

[OR13]

If a test confirms errors are thrown for compressed points, this issue can be closed.