refactor(radio): remove legacy property and support for legacy syntax (#29038)

BREAKING CHANGE:

The `legacy` property and support for the legacy syntax, which involved placing an `ion-radio` inside of an `ion-item` with an `ion-label`, have been removed from radio. For more information on migrating from the legacy radio syntax, refer to the [Radio documentation](https://ionicframework.com/docs/api/radio#migrating-from-legacy-radio-syntax).

core/src/components/item/test/disabled/index.html

@@ -43,12 +43,6 @@
             <ion-checkbox disabled checked> Disabled Checkbox </ion-checkbox>
           </ion-item>
 
-          <ion-radio-group value="radio">
-            <ion-item>
-              <ion-radio disabled value="radio"> Disabled Radio </ion-radio>
-            </ion-item>
-          </ion-radio-group>
-
           <ion-item>
             <ion-range disabled value="10">
               <span slot="label">Disabled Range</span>
@@ -61,18 +55,6 @@
             <ion-label>Multiple Input Disabled Items</ion-label>
           </ion-list-header>
 
-          <ion-item>
-            <ion-checkbox aria-label="Checkbox" disabled slot="start"></ion-checkbox>
-            <ion-label>Checkbox + Radio</ion-label>
-            <ion-radio aria-label="Radio" slot="end"></ion-radio>
-          </ion-item>
-
-          <ion-item>
-            <ion-checkbox aria-label="Checkbox" disabled slot="start"></ion-checkbox>
-            <ion-radio aria-label="Radio" slot="start"></ion-radio>
-            <ion-label>Checkbox + Radio</ion-label>
-          </ion-item>
-
           <ion-item>
             <ion-checkbox slot="start" aria-label="Checkbox"></ion-checkbox>
             <ion-label>Checkbox + Range</ion-label>

core/src/components/item/test/legacy/disabled/index.html

@@ -43,13 +43,6 @@
             <ion-checkbox disabled checked slot="start" legacy="true"></ion-checkbox>
           </ion-item>
 
-          <ion-radio-group value="radio">
-            <ion-item>
-              <ion-label>Disabled Radio</ion-label>
-              <ion-radio disabled value="radio" slot="start" legacy="true"></ion-radio>
-            </ion-item>
-          </ion-radio-group>
-
           <ion-item>
             <ion-label>Disabled Range</ion-label>
             <ion-range disabled value="10" legacy="true"></ion-range>
@@ -63,14 +56,7 @@
 
           <ion-item>
             <ion-checkbox disabled slot="start" legacy="true"></ion-checkbox>
-            <ion-label>Checkbox + Radio</ion-label>
-            <ion-radio slot="end"></ion-radio>
-          </ion-item>
-
-          <ion-item>
-            <ion-checkbox disabled slot="start" legacy="true"></ion-checkbox>
-            <ion-radio slot="start"></ion-radio>
-            <ion-label>Checkbox + Radio</ion-label>
+            <ion-label>Checkbox</ion-label>
           </ion-item>
 
           <ion-item>