Make use of TAG_PUBKEY constants in secp256k1_eckey_pubkey_parse

Summary:
This results in more self-documenting code.

This is a backport of secp256k1's PR562

Test Plan:

make check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4983