remove undefined refs for now

Author: Maximilian-Stefan-Ernst

docs/src/tutorials/backends/nlopt.md

@@ -2,7 +2,7 @@
 
 When [`NLopt.jl`](https://github.com/jump-dev/NLopt.jl) is loaded in the running Julia session,
 it could be used by the [`SemOptimizer`](@ref) by specifying `engine = :NLopt`
-(see [NLopt-specific options](@ref `SemOptimizer(Val(:NLopt))`)).
+(see ...).
 Among other things, `NLopt` enables constrained optimization of the SEM models, which is
 explained in the [Constrained optimization](@ref) section.
 

docs/src/tutorials/backends/optim.md

@@ -1,7 +1,7 @@
 # Using Optim.jl
 
 [Optim.jl](https://github.com/JuliaNLSolvers/Optim.jl) is the default optimization engine of *SEM.jl*,
-see [`SemOptimizer(Val(:Optim))`](@ref) for a full list of its parameters.
+see ... for a full list of its parameters.
 It defaults to the LBFGS optimization, but we can load the `Optim` and `LineSearches` packages
 and specify BFGS (!not L-BFGS) with a back-tracking linesearch and Hager-Zhang initial step length guess: