Remove 1.6-era override mechanism. (#241)

src/device/utils.jl

@@ -1,28 +1,15 @@
 # local method table for device functions
-@static if isdefined(Base.Experimental, Symbol("@overlay"))
 Base.Experimental.@MethodTable(method_table)
-else
-const method_table = nothing
-end
-
-# list of overrides (only for Julia 1.6)
-const overrides = Expr[]
 
 macro device_override(ex)
     ex = macroexpand(__module__, ex)
     if Meta.isexpr(ex, :call)
         ex = eval(ex)
         error()
     end
-    code = quote
+    esc(quote
         $GPUCompiler.@override($method_table, $ex)
-    end
-    if isdefined(Base.Experimental, Symbol("@overlay"))
-        return esc(code)
-    else
-        push!(overrides, code)
-        return
-    end
+    end)
 end
 
 macro device_function(ex)

src/initialization.jl

@@ -44,15 +44,9 @@ function __init__()
         push!(Base.active_repl_backend.ast_transforms, synchronize_metal_tasks)
     end
 
-    # register device overrides
-    eval(Expr(:block, overrides...))
-    empty!(overrides)
-
     @static if !isdefined(Base, :get_extension)
         @require SpecialFunctions = "276daf66-3868-5448-9aa4-cd146d93841b" begin
             include("../ext/SpecialFunctionsExt.jl")
-            eval(Expr(:block, overrides...))
-            empty!(overrides)
         end
     end
 end