[AArch64] Remove dead code for inserting SEH opcodes for realignment. NFC.

If the stack is realigned, we've emitted a frame pointer and
already terminated the SEH prologue, making this dead code since
a07787c9.

The immediate to this SEH opcode was entirely bogus - we don't
know how many bytes the AND operation adjusts the SP, and by
doing "NumBytes & andMaskEncoded" (where andMaskEncoded was the
immediate bitpattern for the AND instruction), the immediate to the
opcode was total gibberish.

This hasn't had any practical effect, since the original stack
pointer always was restored from the frame pointer afterwards anyway.

Differential Revision: https://reviews.llvm.org/D135815