Revert "Fix a crash (assert) when client set serial version < 24 in INIT...

Revert "Fix a crash (assert) when client set serial version < 24 in INIT command SER_FMT_VER_LOWEST is set to zero, then the test is stupid in INIT because all client works. In mapblock we check if client's serialization version is < 24, but if client sent serialization version < 24 (15 for example) the server set it and tried to send nodes, then BOOM"

nerzhul is (may?) be working on a new solution. For the moment this is reverted because old worlds cannot be loaded (see discussion on github for the commit).

This reverts commit 800d1927.