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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224061 91177308-0d34-0410-b5e6-96231b3b80d8