Remove ez-conf-lib:exe variable

[gridbugs]

This package used to provide a variable exe containing the absolute path to the ez-conf-lib script inside the current opam switch. The problem with using an absolute path here is it means the switch is not portable. This causes problems for dune packag emanagement where all packages are first installed to a temporary sandbox and then moved to a final location afterwards.

Clients of ez-conf-lib are updated to refer to the script without using the exe variable in: ocaml/opam-repository#27759

[nberth]

I'm now wondering, this PR is just about removing a variable that would be totally fine to use outside of dune package management. Removing the variable does not appear to be a requirement for ocaml/opam-repository#27759.
Considering this, I think I'd rather keep that feature in and just add a note in the README about that, as it's not an issue when just using opam instead of dune package management (that I actually don't know much about).

[nberth]

@gridbugs I'd just go with #7 instead, unless keeping the package variable is still an issue (for cache management?).

[gridbugs]

Ok yeh happy to go with #7 for now. Interesting point about cache management. I think it will be fine, since I believe dune uses hashes of build rules as keys in its cache, and assumes the contents of files (generated or static) are a pure function of the build rules. This is of course patently untrue in some situations (such as this one) however I think this case is benign provided ocaml/opam-repository#27759 is merged.