build: remove mingw linker workaround from win gitian descriptor
build: remove mingw linker workaround from win gitian descriptor

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

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