Drop obsolete annotations syntax

compiler/testData/psi/packages/PackageModifiers.kt

@@ -1 +1 @@
-public [a] package name
+public @[a] package name

compiler/testData/psi/packages/PackageModifiers.txt

@@ -4,6 +4,7 @@ JetFile: PackageModifiers.kt
       PsiElement(public)('public')
       PsiWhiteSpace(' ')
       ANNOTATION
+        PsiElement(AT)('@')
         PsiElement(LBRACKET)('[')
         ANNOTATION_ENTRY
           CONSTRUCTOR_CALLEE
@@ -18,4 +19,4 @@ JetFile: PackageModifiers.kt
     REFERENCE_EXPRESSION
       PsiElement(IDENTIFIER)('name')
   IMPORT_LIST
-    <empty list>
+    <empty list>