[docs] remove experimental warning from Nonlinear module docstring

Author: blegat

src/Nonlinear/Nonlinear.jl

@@ -4,14 +4,6 @@
 # Use of this source code is governed by an MIT-style license that can be found
 # in the LICENSE.md file or at https://opensource.org/licenses/MIT.
 
-"""
-    Nonlinear
-
-!!! warning
-    The Nonlinear submodule is experimental. Until this message is removed,
-    breaking changes may be introduced in any minor or patch release of
-    MathOptInterface.
-"""
 module Nonlinear
 
 import ForwardDiff