This is not actually used for anything. The configure check causes the
CPU feature flag to be set, but nothing consumes it at all.

While AArch64 does have VFP, it is only used for the scalar C code.
Conversely, it is still possible to disable VFP, by changing the
C compiler flags as before (though that only makes sense for an
hypothetical non-standard Armv8 platform without VFP).

Note that this retains the "vfp" option flag, for backward
compatibility and on the very remote but theoretically possible chance
that FFmpeg actually makes use of it in the future.

AV_CPU_FLAG_VFP is retained as it is actually used by AArch32.