Don't return a value from assertNotNull

Because of kotlin smart casts, there is no need to return a
non-nullable value from `assertNotNull` methods

Issue: junit-team#1866

junit-jupiter-api/src/main/kotlin/org/junit/jupiter/api/Assertions.kt

@@ -179,13 +179,12 @@ fun assertNull(actual: Any?, messageSupplier: () -> String) {
  * @see Assertions.assertNotNull
  */
 @API(since = "5.10", status = EXPERIMENTAL)
-fun <T> assertNotNull(actual: T?): T {
+fun assertNotNull(actual: Any?) {
     contract {
         returns() implies (actual != null)
     }
 
     Assertions.assertNotNull(actual)
-    return actual!!
 }
 
 /**
@@ -201,13 +200,12 @@ fun <T> assertNotNull(actual: T?): T {
  * @see Assertions.assertNotNull
  */
 @API(since = "5.10", status = EXPERIMENTAL)
-fun <T> assertNotNull(actual: T?, message: String): T {
+fun assertNotNull(actual: Any?, message: String) {
     contract {
         returns() implies (actual != null)
     }
 
     Assertions.assertNotNull(actual, message)
-    return actual!!
 }
 
 /**
@@ -223,15 +221,14 @@ fun <T> assertNotNull(actual: T?, message: String): T {
  * @see Assertions.assertNotNull
  */
 @API(since = "5.10", status = EXPERIMENTAL)
-fun <T> assertNotNull(actual: T?, messageSupplier: () -> String): T {
+fun assertNotNull(actual: Any?, messageSupplier: () -> String) {
     contract {
         returns() implies (actual != null)
 
         callsInPlace(messageSupplier, AT_MOST_ONCE)
     }
 
     Assertions.assertNotNull(actual, messageSupplier)
-    return actual!!
 }
 
 /**