remove not race condition aware method in term import

In the term import a method was used for term mid calculation (when no one was given in the TBX) which was

a) not working in mysql 8 out of the box

b) was not save against race conditions and may produce the same mid for different terms.

The term import was modified in a way that it calculates its mids on its own by first inserting the data then updating the mid.