drop `--enable-openssl-tests` from libsecp256k1 config

[whitslack]

Quoting jgriffiths:

--enable-openssl-tests is no longer supported in secp and should be removed

Suggested-by: @jgriffiths See: https://github.com/ElementsProject/libwally-core/pull/414#discussion_r1312347724

[jgriffiths]

Merged, thanks!