Simplify diagnostic message for ACTUAL_MISSING

#KT-21939 Fixed

idea/testData/multiModuleQuickFix/addActualToClass/jvm/jvm.kt

@@ -1,4 +1,4 @@
 // "Add 'actual' modifier" "true"
-// ERROR: Declaration should be marked with 'actual' (suppress with -Xno-check-actual)
+// ERROR: Declaration must be marked with 'actual'
 
 class <caret>Foo

idea/testData/multiModuleQuickFix/addActualToClass/jvm/jvm.kt.after

@@ -1,4 +1,4 @@
 // "Add 'actual' modifier" "true"
-// ERROR: Declaration should be marked with 'actual' (suppress with -Xno-check-actual)
+// ERROR: Declaration must be marked with 'actual'
 
 actual class Foo

idea/testData/multiModuleQuickFix/addActualToClassMember/jvm/jvm.kt

@@ -1,5 +1,5 @@
 // "Add 'actual' modifier" "true"
-// ERROR: Declaration should be marked with 'actual' (suppress with -Xno-check-actual)
+// ERROR: Declaration must be marked with 'actual'
 
 actual class Foo {
     fun <caret>foo() {}

idea/testData/multiModuleQuickFix/addActualToClassMember/jvm/jvm.kt.after

@@ -1,5 +1,5 @@
 // "Add 'actual' modifier" "true"
-// ERROR: Declaration should be marked with 'actual' (suppress with -Xno-check-actual)
+// ERROR: Declaration must be marked with 'actual'
 
 actual class Foo {
     actual fun foo() {}

idea/testData/multiModuleQuickFix/addActualToTopLevelMember/jvm/jvm.kt

@@ -1,4 +1,4 @@
 // "Add 'actual' modifier" "true"
-// ERROR: Declaration should be marked with 'actual' (suppress with -Xno-check-actual)
+// ERROR: Declaration must be marked with 'actual'
 
 fun <caret>foo() {}

idea/testData/multiModuleQuickFix/addActualToTopLevelMember/jvm/jvm.kt.after

@@ -1,4 +1,4 @@
 // "Add 'actual' modifier" "true"
-// ERROR: Declaration should be marked with 'actual' (suppress with -Xno-check-actual)
+// ERROR: Declaration must be marked with 'actual'
 
 actual fun foo() {}