Drop obsolete annotations syntax

compiler/testData/psi/secondaryConstructors/basic.kt

@@ -5,11 +5,11 @@ class A {
 
     private annot constructor(x: Int) {}
 
-    [constructor] fun constructor() {}
+    @constructor fun constructor() {}
 
     annot protected constructor(x: Int, y: Int) : this(1,2) {}
 
-    [constructor] public constructor() : super() {
+    @[constructor] public constructor() : super() {
         x = 1
     }
 }

compiler/testData/psi/secondaryConstructors/basic.txt

@@ -68,15 +68,13 @@ JetFile: basic.kt
       PsiWhiteSpace('\n\n    ')
       FUN
         MODIFIER_LIST
-          ANNOTATION
-            PsiElement(LBRACKET)('[')
-            ANNOTATION_ENTRY
-              CONSTRUCTOR_CALLEE
-                TYPE_REFERENCE
-                  USER_TYPE
-                    REFERENCE_EXPRESSION
-                      PsiElement(IDENTIFIER)('constructor')
-            PsiElement(RBRACKET)(']')
+          ANNOTATION_ENTRY
+            PsiElement(AT)('@')
+            CONSTRUCTOR_CALLEE
+              TYPE_REFERENCE
+                USER_TYPE
+                  REFERENCE_EXPRESSION
+                    PsiElement(IDENTIFIER)('constructor')
         PsiWhiteSpace(' ')
         PsiElement(fun)('fun')
         PsiWhiteSpace(' ')
@@ -146,6 +144,7 @@ JetFile: basic.kt
       SECONDARY_CONSTRUCTOR
         MODIFIER_LIST
           ANNOTATION
+            PsiElement(AT)('@')
             PsiElement(LBRACKET)('[')
             ANNOTATION_ENTRY
               CONSTRUCTOR_CALLEE