[electrum] remove misleading LIBSECP_COMMIT var

Summary: We build libsecp from the latest master branch on the github repo. This variable has been unused for a very long time and is misleading.

Test Plan: grep -r "LIBSECP_COMMIT" .

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16727