Ipopt executables from AMPL now require an AMPL login

- and I haven't checked whether you can still find a download after login

README.md

@@ -113,7 +113,6 @@ Some precompiled binaries of Ipopt are also available:
 - **[Ipopt releases page](https://github.com/coin-or/Ipopt/releases)** provides libraries and executables for Windows
 - **[JuliaBinaryWrappers](https://github.com/JuliaBinaryWrappers/Ipopt_jll.jl/releases)** provides libraries and executables
 - **[IDEAS](https://github.com/IDAES/idaes-ext/releases)** provides executables; these executables include HSL solvers
-- **[AMPL](http://ampl.com/products/solvers/open-source/#ipopt)** provides executables
 - **[Pardiso project](https://pardiso-project.org/index.html#binaries)** provides libraries for using Ipopt with Pardiso through Matlab (Matpower)
 
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