refactor[ux]: remove deprecated VyperNode properties

[tserg]

What I did

Depends on #3998.

Remove deprecated properties from Num and Str vyper nodes following in the spirit of the equivalent deprecated python AST classes.

How I did it

How to verify it

Commit message

This commit removes the `n` property from `Num` vyper node and the `s`
property from `Str` vyper node as they have been deprecated in the
python AST (and it is clearer to use the `value` property anyways)

Description for the changelog

Cute Animal Picture

[charles-cooper]

@tserg this is ready for review right?

[tserg]

@tserg this is ready for review right?

Yes, just pulled the latest master.