Revert "Add GMP dependency."
    
    This causes issues with building other ruby packages, since it would
    force every package to depend on gmp-devel. Reverting this until this
    gets resolved upstream:
    
    https://bugs.ruby-lang.org/issues/14422#note-6
    
    This reverts commit 5624addc78a1a8b4a4d5dd31d76624249ce3a46d.