[BUILD] Search and include OpenSSL only where required

Summary:
Since D6242 there is no more OpenSSL needed outside of the BIP70
feature. This diff cleans up the build system to reflect this.

Inspired by
https://github.com/bitcoin/bitcoin/pull/17165/commits/fcee10c2d028cba11416d902f5abf13fea7a65f4
and https://github.com/bitcoin/bitcoin/pull/17265/commits/8983ee3e6dd8ab658bd2caf97c326cc53ea50818,
but obviously different because we maintain BIP70.

Depends on D6251.

Test Plan:

ninja all check-all bench-bitcoin

make
make check

Run the Gitian builds.

Uninstall OpenSSL (or make it fail to be found), then:

cmake -GNinja .. \
  -DENABLE_BIP70=OFF \
  -DSECP256K1_BUILD_OPENSSL_TESTS=OFF
ninja all check-all

With autotools, the whole secp256k1 test suite should be disabled in
order to skip openssl detection. Unfortunately the option name collides
with the bitcoin abc option to disable the tests, so they end up to be
all disabled.

../configure \
  --enable-deprecated-build-system \
  --disable-bip70 \
  --enable-tests=no
make

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6243