This reverts commit 9dc93220c1.
On one hand, conflicting definitions for `sshAuthSock.initialization.*`
were triggering before the assertion triggered, hereby making the
assertion redundant. On the other hand, the assertion prevents users
from running multiple agents side-by-side even after resolving the
definition conflict.
See https://github.com/nix-community/home-manager/pull/8533#issuecomment-4224165151.