Remove IceTest.cpp and its corresponding subzerotest binary.

By now, the IceTest functionality is completely subsumed by llvm2ice.

    subzerotest cond
    subzerotest loop
are now equivalent to:
    llvm2ice -verbose=all tests_lit/llvm2ice_tests/simple-cond.ll
    llvm2ice -verbose=all tests_lit/llvm2ice_tests/simple-loop.ll

In virtually all cases, it should be easier to add any missing
functionality to llvm2ice than to hack on subzerotest.