Delete old != method for ReturnCode

[JamesWrigley]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

It causes invalidations and this odd behaviour without the corresponding == method:

julia> ReturnCode.Success == :Success
false

julia> ReturnCode.Success != :Success
false

Should fix these invalidations seen when loading CurveFit.jl:

 inserting !=(retcode::SciMLBase.ReturnCode.T, s::Symbol) @ SciMLBase ~/.julia/packages/SciMLBase/tmJhj/src/retcodes.jl:442 invalidated:                                                                                                       
   mt_backedges: 1: signature Tuple{typeof(!=), Any, Any} triggered MethodInstance for canonicalize(::Dates.CompoundPeriod) (1 children)                                                                                                       
   backedges: 1: superseding !=(x, y) @ Base operators.jl:321 with MethodInstance for !=(::Any, ::Any) (235 children) 

Also see: #393

[JamesWrigley]

BTW would it be possible to tag a new release with this?