Revert "[ARM] Allow D-reg copies to use VMOVD with fpregs64"

This reverts commit 0a762ec1.

Some CPUs enable fp64 by default (such as cortex-m7). When specifying a
single-precision fpu with them like -mfpu=fpv5-sp-d16, the fp64 feature will
be disabled, but fpreg64 will not. We need to disable them both correctly under
clang in order for the backend to be able to use the reliably. In the meantime
this reverts 0a762ec1 until that issue is fixed.