remove dead store in ecdsa_signature_parse_der_lax

Summary:
This change was made in bitcoin/bitcoin without upstreaming. So this is
a followup to the comment here:
https://github.com/bitcoin/bitcoin/pull/19228#issuecomment-641795558.

See also: https://github.com/bitcoin/bitcoin/pull/11073.

This is a backport of libsecp256k1 PR765

Test Plan:

ninja check-secp256k1

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D7592