CodeGen: Stop using LeakDetector for MachineInstr

Since `MachineInstr` is required to have a trivial destructor, it cannot
remove itself from `LeakDetection`.  Remove the calls.

As it happens, this requirement is because `MachineFunction` allocates
all `MachineInstr`s in a custom allocator; when the `MachineFunction` is
destroyed they're dropped of the edge.  There's no benefit to detecting
leaks.