Remove unused `include AST::Util`

src/ameba/rule/lint/empty_expression.cr

@@ -28,8 +28,6 @@ module Ameba::Rule::Lint
   #   Enabled: true
   # ```
   class EmptyExpression < Base
-    include AST::Util
-
     properties do
       description "Disallows empty expressions"
     end

src/ameba/rule/lint/unreachable_code.cr

@@ -42,8 +42,6 @@ module Ameba::Rule::Lint
   #   Enabled: true
   # ```
   class UnreachableCode < Base
-    include AST::Util
-
     properties do
       description "Reports unreachable code"
     end

src/ameba/rule/lint/unused_block_argument.cr

@@ -31,8 +31,6 @@ module Ameba::Rule::Lint
   #   Enabled: true
   # ```
   class UnusedBlockArgument < Base
-    include AST::Util
-
     properties do
       description "Disallows unused block arguments"
     end