Remove configure-time check for `clang-format'.

The version of `clang-format' that comes with the LLVM/clang 3.3 release
doesn't work so well for us.  (The problems with `clang-format' are
apparently fixed in versions shortly after the LLVM/clang 3.3 release,
but at the moment, we are trying to target the 3.3 release.)