core: Remove TestContext2Validate_PlanGraphBuilder

This test seems to be a holdover from the many-moons-ago switch from one
graph for all operations to separate graphs for plan and apply. It is
effectively just a copy of a subset of the content of the Context.Validate
function and is a maintainability hazard because it tends to lag behind
updates to that function unless changes there happen to make it fail.

This test doesn't cover anything that the other validate context tests
don't exercise as an implementation detail of calling Context.Validate,
so I've just removed it with no replacement.

internal/terraform/context_validate_test.go

@@ -1187,32 +1187,6 @@ resource "aws_instance" "foo" {
 	}
 }
 
-// Manually validate using the new PlanGraphBuilder
-func TestContext2Validate_PlanGraphBuilder(t *testing.T) {
-	fixture := contextFixtureApplyVars(t)
-	opts := fixture.ContextOpts()
-	c := testContext2(t, opts)
-
-	graph, diags := ValidateGraphBuilder(&PlanGraphBuilder{
-		Config:  fixture.Config,
-		State:   states.NewState(),
-		Plugins: c.plugins,
-	}).Build(addrs.RootModuleInstance)
-	if diags.HasErrors() {
-		t.Fatalf("errors from PlanGraphBuilder: %s", diags.Err())
-	}
-	defer c.acquireRun("validate-test")()
-	walker, diags := c.walk(graph, walkValidate, &graphWalkOpts{
-		Config: fixture.Config,
-	})
-	if diags.HasErrors() {
-		t.Fatal(diags.Err())
-	}
-	if len(walker.NonFatalDiagnostics) > 0 {
-		t.Fatal(walker.NonFatalDiagnostics.Err())
-	}
-}
-
 func TestContext2Validate_invalidOutput(t *testing.T) {
 	m := testModuleInline(t, map[string]string{
 		"main.tf": `