Performing a series of link checks for a Boost component that is
header-only doesn't make much sense, and currently means we just have
another confusing Boost macro in our tree. I'm not sure why this was
originally done this way; maybe Sjors or luke-jr can elaborate
(#15382 (929cda5470))?
The macro also has the side-effect of producing confusing error
messages. i.e in #20744, the CI is currently failing with:
```bash
checking for boostlib >= 1.58.0 (105800) lib path in "/tmp/cirrus-ci-build/depends/x86_64-pc-linux-gnu/lib"... yes
checking for boostlib >= 1.58.0 (105800)... yes
checking whether the Boost::Process library is available... yes
configure: error: Could not find a version of the Boost::Process library!
```
This isn't useful, given there is no such thing as a `Boost::Process`
library.
This PR just removes the macro entirely, but maintains a `--with-boost-process`
(defaulting to off), flag to configure. Hopefully this will also be
removed, in favour of `--enable-disable-external-signer` if/when #16546
is merged.
fanquake