Remove Julia v1.9 get_extension compatibility code

Since all SciML packages now require Julia v1.10+, we can remove the
compatibility checks for `isdefined(Base, :get_extension)` as package
extensions are available in all supported Julia versions.

This removes unnecessary version checks in extension loading code.

Author: ChrisRackauckas

src/ADTypes.jl

@@ -26,11 +26,6 @@ include("sparse.jl")
 include("legacy.jl")
 include("symbols.jl")
 
-if !isdefined(Base, :get_extension)
-    include("../ext/ADTypesChainRulesCoreExt.jl")
-    include("../ext/ADTypesEnzymeCoreExt.jl")
-end
-
 # Automatic Differentiation
 export AbstractADType
 export AutoChainRules,