unattended-upgrade最近自动将我的内核升级到 4.4.0-143，但它不适用于 Nvidia 显卡，所以我决定回滚到以前的版本。
我能够启动到较旧的内核（4.4.0-141），并确认我正在启动我的计算机：
4.4.0-141-generic
但是当我尝试时apt purge linux-image-4.4.0-143-generic，apt 提示我确认：
The following NEW packages will be installed:
linux-image-unsigned-4.4.0-143-generic
那么为什么卸载一个签名的内核需要安装一个未签名的版本呢？我怎样才能安全地删除较新的（143）内核？提前致谢！
apt purge 的完整输出：
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages were automatically installed and are no longer required:
amd64-microcode intel-microcode iucode-tool linux-headers-4.4.0-137 linux-headers-4.4.0-137-generic linux-headers-4.4.0-138 linux-headers-4.4.0-138-generic linux-headers-4.4.0-139 linux-headers-4.4.0-139-generic linux-headers-4.4.0-141 linux-headers-4.4.0-141-generic
linux-image-4.4.0-137-generic linux-image-4.4.0-138-generic linux-image-4.4.0-139-generic linux-image-4.4.0-141-generic linux-image-extra-4.4.0-137-generic linux-image-extra-4.4.0-138-generic linux-image-extra-4.4.0-139-generic linux-image-extra-4.4.0-141-generic
linux-signed-image-4.4.0-137-generic linux-signed-image-4.4.0-138-generic linux-signed-image-4.4.0-139-generic linux-signed-image-4.4.0-141-generic thermald
Use 'sudo apt autoremove' to remove them.
The following additional packages will be installed:
linux-image-unsigned-4.4.0-143-generic
Suggested packages:
fdutils linux-doc-4.4.0 | linux-source-4.4.0 linux-tools
The following packages will be REMOVED:
linux-generic* linux-image-4.4.0-143-generic* linux-image-generic* linux-signed-generic*
The following NEW packages will be installed:
linux-image-unsigned-4.4.0-143-generic
0 upgraded, 1 newly installed, 4 to remove and 140 not upgraded.
Need to get 7,049 kB of archives.
After this operation, 386 kB of additional disk space will be used.
Do you want to continue? [Y/n] n