Fix warning about collect_delay during precompile (#4484)

Author: Keno

src/REPLMode/command_declarations.jl

@@ -512,7 +512,7 @@ compound_declarations = [
             :name => "gc",
             :api => API.gc,
             :option_spec => [
-                PSA[:name => "all", :api => :collect_delay => Hour(0)],
+                PSA[:name => "all", :api => :collect_delay => nothing],
                 PSA[:name => "verbose", :short_name => "v", :api => :verbose => true],
             ],
             :description => "garbage collect packages not used for a significant time",

test/new.jl

@@ -2765,7 +2765,7 @@ end
         @test isempty(opts)
         api, opts = first(Pkg.pkg"gc --all")
         @test api == Pkg.gc
-        @test opts[:collect_delay] == Hour(0)
+        # N.B.: `--all` is now a no-op, but is retained for now for compatibility.
     end
 end