Revert "Base.get_extension made public" (#59593)

Reverts #58108. It is not allowed for packages to call
`Base.get_extension`, so it should never have been made public.
Something simply being documented in the manual is never any allowable
argument for making something public.

The correct way to use this functionality is with dispatch. Loosely:
```julia
module Main
function get_extension end
end

module MainPkgExt # extension module in Main
using Other
Main.get_extension(args...) = Other.call(args...)
end
```

Author: vtjnash

base/public.jl

@@ -54,7 +54,6 @@ public
     active_project,
 
 # Reflection and introspection
-    get_extension,
     isambiguous,
     isexpr,
     isidentifier,