refactor(toolbar): improve slot layout and visibility handling (#30371)

Co-authored-by: ionitron <hi@ionicframework.com>
Co-authored-by: Brandy Smith <brandyscarney@users.noreply.github.com>

core/src/css/ionic/core.ionic.scss

@@ -71,23 +71,6 @@ html.ionic ion-modal ion-toolbar {
   --padding-end: calc(var(--ion-safe-area-left) + #{globals.$ion-space-400});
 }
 
-/**
-* Center an `ion-title` within a modal header. This works around
-* limitations with `:host-context()` not being supported in all browsers.
-*/
-html.ionic ion-modal ion-header ion-toolbar ion-title {
-  @include globals.position(0, null, null, 0);
-
-  position: absolute;
-
-  width: 100%;
-  height: 100%;
-
-  transform: translateZ(0);
-
-  text-align: center;
-}
-
 /**
 * Add padding to the left, right, and bottom of `ion-content`
 * within a modal.