libct/user: remove

This package is marked as deprecated in v1.2.0, and have no internal
users, so let's remove it (for v1.3.0).

Signed-off-by: Kir Kolyshkin <[email protected]>

Author: kolyshkin

CHANGELOG.md

@@ -16,6 +16,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
    user previously relied on this feature, now they have to convert names to
    IDs before calling libcontainer; it is recommended to use Go package
    github.com/moby/sys/user for that. (#3999)
+ * Previously deprecated libcontainer/user package is removed; use
+   github.com/moby/sys/user instead. (#3999)
 
 ## [1.2.0] - 2024-10-22