Remove note about heap allocation in secp256k1_ecmult_odd_multiples_table_storage_var

Summary:
which was removed in 47045270fa90f81205d989f7107769bce1e71c4d

This is a backport of libsecp256k1 PR677

Test Plan:

ninja check-secp256k1

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5366