Introduce Variable: Remove braces from replaced string template entries when possible

#KT-5310 Fixed

idea/testData/refactoring/introduceVariable/OccurrencesInStringTemplate.kt.after

@@ -1,5 +1,5 @@
 fun a(x: Int) {
     val i = x + 1
     val a = i
-    "it's a number ${i}"
+    "it's a number $i"
 }

idea/testData/refactoring/introduceVariable/StringInjection.kt.after

@@ -1,4 +1,4 @@
 fun a() {
     val i = 1
-    "it's a number ${i}"
+    "it's a number $i"
 }