Remove stale file i386-avx.dat

I noticed that regformats/i386/i386-avx.dat did not get re-generated
when doing "make" in the features directory.  I think it's a leftover
from commit

  f5a29eb0 ("Clean up x86 non-linux GDBserver target descriptions")

I build-tested gdbserver with amd64 and i386.

gdb/ChangeLog:

	* regformats/i386/i386-avx.dat: Remove.