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.