build: remove mingw linker workaround from win gitian descriptor

Summary:

This workaround was added as part of the switch to gitian building using Ubuntu 14.04 (#6900).
However, it should no longer be required, as we have switched to Bionic (#13171), and that
has a far newer version of binutils.

binutils patch: https://sourceware.org/bugzilla/show_bug.cgi?id=16192

Bitcoin ABC now uses Debian Buster, which has an even more recent version of binutils (2.31 vs 2.30).

This is a backport of core#16667

Depends on D10584

Test Plan: @bot gitian-win

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10587