Drop support for MTL expressions

Follow up of #407  (drop support of OCL) and #68 (move to AQL 8).

We support using MTL ([/]) expressions in VSMs through org.eclipse.sirius.common.acceleo.
The MTL language comes from Acceleo 3.
We used to support it alongside AQL 7 (from Acceleo 3.7 branch)

As we moved to AQL 8 (from the Acceleo 4 branch), we would like to drop support of MTL expressions. 