qemu: Always assume support for QEMU_CAPS_CPU_CACHE

qemu: Always assume support for QEMU_CAPS_CPU_CACHE

All supported qemu versions have this feature and it can't be compiled
out. The logic is a bit more complex in this instance as the flag is
asserted if:

    if (ARCH_IS_X86(qemuCaps->arch) &&
        virQEMUCapsGet(qemuCaps, QEMU_CAPS_QUERY_CPU_MODEL_EXPANSION)) {
        virQEMUCapsSet(qemuCaps, QEMU_CAPS_CPU_CACHE);
    }

Now QEMU_CAPS_QUERY_CPU_MODEL_EXPANSION is available since qemu-2.8 but
only on certain architectures, thus we need to keep the flag itself, but
x86_64 is one of them.

The flag can be also assumed as qemuValidateDomainDefCpu rejects any
cache config on non-x86 arches.

Remove any use of the capability and drop the impossible test cases.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>