[Build]: Use one debian mirror config (#10281)

Why I did it
Use one debian mirror config.
The empty config in https://github.com/Azure/sonic-buildimage/blob/master/files/image_config/apt/sources.list overrides the file https://github.com/Azure/sonic-buildimage/blob/master/files/apt/sources.list.amd64 (armhf/arm64), it does not make sense.
All the content in files/image_config/apt is no use, any one wants to add mirror config, please add in files/apt.

How I did it
Remove files/image_config/apt and the reference.

Author: xumia

files/build_templates/sonic_debian_extension.j2

@@ -68,15 +68,6 @@ else
     sudo chroot $FILESYSTEM_ROOT service docker start
 fi
 
-# Apply apt configuration files
-sudo cp $IMAGE_CONFIGS/apt/sources.list $FILESYSTEM_ROOT/etc/apt/
-sudo mkdir -p $FILESYSTEM_ROOT/etc/apt/sources.list.d/
-sudo cp -R $IMAGE_CONFIGS/apt/sources.list.d/${CONFIGURED_ARCH}/* $FILESYSTEM_ROOT/etc/apt/sources.list.d/
-cat $IMAGE_CONFIGS/apt/sonic-dev.gpg.key | sudo LANG=C chroot $FILESYSTEM_ROOT apt-key add -
-
-# Update apt's snapshot of its repos
-sudo LANG=C DEBIAN_FRONTEND=noninteractive chroot $FILESYSTEM_ROOT apt-get update
-
 # Apply environtment configuration files
 sudo cp $IMAGE_CONFIGS/environment/environment $FILESYSTEM_ROOT/etc/
 sudo cp $IMAGE_CONFIGS/environment/motd $FILESYSTEM_ROOT/etc/