[New msys2-runtime-package version] Remove all cygwin CHOST changes

[github-actions]

Remove all cygwin CHOST changes

Since #5448 CHOST defaults to cygwin now, so this
is no longer needed.

msys2/MSYS2-packages@b38b2b7

[dscho]

We could probably do that in Git for Windows' msys2-runtime builds because that package cannot be built for i686 anymore, anyway. @rimrul do you have opinions?

[rimrul]

Our msys2-runtime package doesn't seem to use CYGWIN_CHOST anywhere in its PKGBUILD.

[dscho]

@rimrul true! Git for Windows' MSYS2-packages/msys2-runtime/PKGBUILD only mentions HOST and therefore does not require that change. Thank you for spotting that!