Revert removal of deprecated LagrangeSimplexLFE constructor and undeprecate it

Commit cd117ede removed this constructor. The reason of its deprecation seems to be a mistake. The message pointing at the deprecation apparently was introduced when an alternative constructor existed in f4655053. However, the deprecation warning was shortly removed in 50cdcd56, but it forgot to remove the message from the previous commit (both commits were done the same day). With no alternative constructor and the history of the comment, this deprecation comment seems to be void.

Note that the MR introducing the removal in !294 (merged) broke the PDELab pipeline as it uses the removed constructor. There seems to be not an straight forward alternative to it.