EDAC/amd64: Do not warn when removing instances

On machines which do not populate all nodes with DIMMs, the driver
doesn't initialize an instance there. However, the instance removal
remove_one_instance() path will warn unconditionally, which is wrong.

Remove the WARN_ON() even if the warning is innocent because it causes a
splat in dmesg.

Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20200117115939.5524-1-bp@alien8.de