MINOR: connection: simplify removal of idle conns from their trees

MINOR: connection: simplify removal of idle conns from their trees

Small change of API for conn_delete_from_tree(). Now the connection
instance is taken as argument instead of its inner node.

No functional change introduced with this commit. This simplifies
slightly invocation of conn_delete_from_tree(). The most useful changes
is that this function will be extended in the next patch to be able to
remove the connection from its new idle list at the same time as in its
idle tree.