powerpc/module: Remove arch specific module bug stuff

The last function to reference module_bug_list went in 2008's
  commit b9754568 ("powerpc: Remove dead module_find_bug code")
but I don't think that was called since 2006's
  commit 73c9ceab ("[POWERPC] Generic BUG for powerpc")

Now that the list has gone, I think we can also clean up the bug
entries in mod_arch_specific.

Lightly boot tested.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240503002317.183500-1-linux@treblig.org