node: The default feature is a footgun

[tcharding]

corepc-node has a default feature 28_0 - this is an unnecessary footgun because it overides the version supplied in another feature e.g., cargo test --features=25_2 runs tests using Core v28 still because default features are enabled.

[tcharding]

Found here: RCasatta/electrsd#89 (comment)

[apoelstra]

Good catch. I guess we should disable all features by default and have a compile error telling users that they need to enable one.