remove dead store in ecdsa_signature_parse_der_lax
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 [[https://github.com/bitcoin-core/secp256k1/pull/765 | PR765]]

Test Plan:
  ninja check-secp256k1

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

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

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