[AArch64][GlobalISel] NFC: Remove dead G_BUILD_VECTOR legalization rule

Remove a rule which allows larger scalar types than the destination vector
element type.

This appears to be irrelevant now that we have G_BUILD_VECTOR_TRUNC. Plus,
making a G_BUILD_VECTOR which satisfies this introduces a verifier failure
anyway.

Differential Revision: https://reviews.llvm.org/D97727