Revert "feat(ssh-agent): only start ssh-agent once (#5359)"

This reverts commit 7692881d.

plugins/ssh-agent/ssh-agent.plugin.zsh

@@ -96,24 +96,7 @@ else
   _start_agent
 fi
 
-() {
-  emulate -L zsh
-
-  command mkdir "$ZSH_CACHE_DIR/ssh-agent.lock" 2>/dev/null || return
-
-  trap "
-    ret=\$?
-
-    command rm -rf '$ZSH_CACHE_DIR/ssh-agent.lock'
-    unset agent_forwarding ssh_env_cache
-    unfunction _start_agent _add_identities 2>/dev/null
-
-    return \$ret
-  " EXIT INT QUIT
-
-  _add_identities
-
-}
+_add_identities
 
 unset agent_forwarding ssh_env_cache
 unfunction _start_agent _add_identities