coin: Drop the vcpkg triplet vars from the Yocto cross configs

qemu-arm64-developer-build and imx8-arm64-developer-build-2404 build
against boot2qt sysroots that already provide the third-party libraries
vcpkg would supply, so vcpkg has no role there.

Both are hosted on Ubuntu 24.04 x86_64, which provisions vcpkg for
x64-linux-qt only. VCPKG_ROOT is therefore set, qt.toolchain.cmake
chainloads vcpkg, and CMAKE_PREFIX_PATH gets an
installed/arm64-linux-qt entry that is not provisioned on the host.
Nothing breaks today because the sysroot satisfies every find_package.

Drop the host triplet too; with no vcpkg role it has nothing to point
at.

Task-number: QTBUG-115715
Pick-to: 6.12
Change-Id: I3b0135c0a663d871c167f2b082cc00236d3c59c2
Reviewed-by: Elias Toivola <elias.toivola@qt.io>