MINOR: quic: simplify the removal from ODCID tree

MINOR: quic: simplify the removal from ODCID tree

With the DCID refactoring, the locking is more centralized. It is
possible to simplify the code for removal of a quic_conn from the ODCID
tree.

This operation can be conducted as soon as the connection has been
retrieved from the DCID tree, meaning that the peer now uses the final
DCID. Remove the bit to flag a connection for removal and just uses
ebmb_delete() on each sucessful lookup on the DCID tree. If the
quic_conn has already been removed, it is just a noop thanks to
eb_delete() implementation.