MIPS: eBPF: Remove REG_32BIT_ZERO_EX

REG_32BIT_ZERO_EX and REG_64BIT are always handled in exactly the same
way, and reg_val_propagate_range() never actually sets any register to
type REG_32BIT_ZERO_EX.

Remove the redundant & unused REG_32BIT_ZERO_EX.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>