Remove configure-time setting of DEFAULT_NATPMP and DEFAULT_UPNP

Summary:
Default to false.

We never set these "default default" values in any build, so we can simplify the configuration.
Note that we could simplify it even further by merging USE_[UPNP,NATPMP] and ENABLE_[UPNP,NATPMP], which are now aliases of each-other, but all of these options are going away soon so it's not worth the extra effort.

This is a backport of core#26896 and core#28874

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D19756