[ARM MVE] Remove accidental 64-bit vst2/vld2 intrinsics.

ACLE defines no such intrinsic as vst2q_u64, and the MVE instruction
set has no corresponding instruction. But I had accidentally added
them to the fledgling <arm_mve.h> anyway, and if you used them, you'd
get a compiler crash.

Reviewers: dmgreen

Subscribers: kristof.beyls, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D69788