KT-6019 Join lines should remove code block in when entry body

#KT-6019 Fixed

idea/testData/joinLines/removeBraces/WhenEntry.kt

@@ -0,0 +1,7 @@
+fun foo(p: Int) {
+    when (p) {
+        1 -> {<caret>
+            println()
+        }
+    }
+}

idea/testData/joinLines/removeBraces/WhenEntry.kt.after

@@ -0,0 +1,5 @@
+fun foo(p: Int) {
+    when (p) {
+        1 -> println()
+    }
+}