Fixing sigsegv

Delete unnecessary class EtherCAT_SlaveDb that was causing sigsegv (when accessing uninitialized elements).

This class wasn't actually being used for any purpose.

I'm not sure if I should delete its declaration, but we definitely should not use it in the `scan_slaves` function, that is only called when initializing the ethercat Application Layer.
This is where the sigsegv happens: `m_sc[i]->is_used()`
Pointers in `m_sc` are not pointing to a valid EtherCAT_SlaveConfig object, as none was ever constructed.

For the same reason EtherCAT_SlaveDb is unnecessary. You will never find anything in an empty DB.