Remove dead i386 assembly code from softmmu_header.h

This patch removes dead i386 assembly code from softmmu_header.h.

The code is conditional on ASM_SOFTMMU, which is never defined.
Optimisation for the fast path is already handled by tcg_out_qemu_ld()
and tcg_out_qemu_st(), so there seems to be little need for this code.

Signed-off-by: Stuart Brady <stuart.brady@gmail.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>