EDAC/amd64: Remove unused struct member amd64_pvt::ext_nbcfg

EDAC/amd64: Remove unused struct member amd64_pvt::ext_nbcfg

Commit

  cfe40fdb4a46 ("amd64_edac: add driver header")

added amd64_pvt struct with ext_nbcfg in it. But no one used that member
since then.

Therefore, remove it.

Found by https://github.com/jirislaby/clang-struct.

Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Yazen Ghannam <yazen.ghannam@amd.com>
Link: https://lore.kernel.org/r/20240213112051.27715-2-jirislaby@kernel.org
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>