ghc-bignum: remove obsolete ln script

This commit removes an obsolete ln script in ghc-bignum/gmp. See
060251c2 for its original intention,
but it's been obsolete for a long time, especially since the removal
of the make build system. Hence the house cleaning.

(cherry picked from commit c62dc317)
(cherry picked from commit 5f4848c0)
(cherry picked from commit 5b9561d6)