Description and problems

- Do not use a description but a Symbol, since it is not clear at all what modeller is the default.

https://github.com/control-toolbox/OptimalControlProblems.jl/blob/7c8abeeb742118ae0b859da94bb1cebec131f969/ext/OptimalControlModels_s/beam_s.jl#L32

- Since the problems with `_s` have more compatibility, they should be the standard problems. Remove `_s` suffix and remove the other problems.