chore: remove references to dbtestutil.WillUsePostgres (#20436)

Addresses https://github.com/coder/internal/issues/758.

This PR only cleans up dead code, it makes no changes to test logic.

enterprise/coderd/workspaceagents_test.go

@@ -89,10 +89,6 @@ func TestBlockNonBrowser(t *testing.T) {
 func TestReinitializeAgent(t *testing.T) {
 	t.Parallel()
 
-	if !dbtestutil.WillUsePostgres() {
-		t.Skip("dbmem cannot currently claim a workspace")
-	}
-
 	if runtime.GOOS == "windows" {
 		t.Skip("test startup script is not supported on windows")
 	}