Remove the last uses of `struct ndpi_id_struct`. That code is not really used and it has not been updated for a very long time: see #1279 for details. Correlation among flows is achieved via LRU caches. This change allows to further reduce memory consumption (see also 91bb77a8). At nDPI 4.0 (more precisly, at a6b10cf, because memory stats were wrong until that commit): ``` nDPI Memory statistics: nDPI Memory (once): 221.15 KB Flow Memory (per flow): 2.94 KB ``` Now: ``` nDPI Memory statistics: nDPI Memory (once): 235.27 KB Flow Memory (per flow): 688 B <-------- ``` i.e. memory usage per flow has been reduced by 77%. Close #1279