*-w64-mingw32-headers: Deactivate *-w64-mingw32-winpthreads

[Gcenx]

Closes https://trac.macports.org/ticket/73308

Description

Type(s)

• bugfix

Tested on

macOS 15.7.2 24G325 x86_64
Xcode 26.1.1 17B100

Verification

• followed our Commit Message Guidelines?
• squashed and minimized your commits?
• checked that there aren't other open pull requests for the same change?
• referenced existing tickets on Trac with full URL in commit message?
• checked your Portfile with port lint?
• tried existing tests with sudo port test?
• tried a full install with sudo port -vst install?
• tested basic functionality of all binary files?
• checked that the Portfile's most important variants haven't been broken?

[Gcenx]

@ryandesign not used the deactivate hack much so might want to review it