remove old conf-gmp packages

[hannesm]

these fail in the lower bounds due to newer C compilers:

# + cc -c -I/usr/local/include test.c
# test.c: In function 'test':
# test.c:7:9: error: implicit declaration of function '__gmp_init'; did you mean '__gmpf_init'? [-Wimplicit-function-declaration]
#     7 |         __gmp_init();
#       |         ^~~~~~~~~~
#       |         __gmpf_init

[jmid]

Yes, please! Should the old versions be conserved by a corresponding PR to the archive repo to preserve balance in the universe? 🤔 (genuine question - I think the case can be made for both preservation and dropping)

[hannesm]

@jmid we can preserve them of course if that is requested by someone.

[mseri]

For consistency's sake, I'd be in favour moving them to the archive in parallel with this PR. @shonfeder and @raphael-proust what do you think?

[raphael-proust]

Also in favour of moving them to the archive. The alternative is to mark them as "avoid version". Either of those is fine with me.

[jmid]

Also in favour of moving them to the archive. The alternative is to mark them as "avoid version". Either of those is fine with me.

Already tried in #28540 and #28545 - yet the lower bounds check still tries them.

[mseri]

See ocaml/opam-repository-archive#28