cmd/cgo: stop using -fno-eliminate-unused-debug-types

This flag was added in January 2010, in CL 181102, to fix issue 497.
(Numbers were just shorter back then.) The fix was for OS X machines
and the llvm-gcc frontend.

In July 2011 we had to change the way we get enum values, because
there were no flags available to force Xcode's llvm-gcc to include the
enum names and values in DWARF debug output.

We now use clang, not llvm-gcc, on OS X machines.
Earlier versions of clang printed a warning about not knowing the flag.
Newer versions of clang now make that an error.

That is:
 - The flag was added for OS X machines.
 - The flag is no longer necessary on OS X machines.
 - The flag now breaks some OS X machines.

Remove it.

I have run the original program from issue 497 successfully
without the flag on both OS X and Linux machines.

Fixes issue 6678.

R=golang-dev, minux.ma
CC=golang-dev
https://codereview.appspot.com/18850043