bluetooth: host: att: Remove meaningless check

`>= 0` was used when EATT support was implemented (#23199) because
`bt_l2cap_chan_send` could return number of bytes sent. After PR #67528,
`bt_l2cap_chan_send` doesn't return amount of bytes sent or any positive
value, but either 0 or negative value. Thus `>= 0` is not needed. It
also confusing when reading code, especially when the same check is not
implemented in other cases where underlying function `chan_send` is
used.

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>