Remove redundant encoding (#614)

This magic comment is not needed anymore because the default encoding of
source code files assumed by Ruby 2 is utf-8, and the minimum required
Ruby version is 2.7

Autofixed with:
```
rubocop -a
```

Author: tagliala

.rubocop.yml

@@ -565,6 +565,9 @@ RSpec/VoidExpect:
 RSpec/Yield:
   Enabled: true
 
+Style/Encoding:
+  Enabled: true
+
 Style/FrozenStringLiteralComment:
   Enabled: true
   Exclude:

arbre.gemspec

@@ -1,4 +1,3 @@
-# -*- encoding: utf-8 -*-
 # frozen_string_literal: true
 $:.push File.expand_path("../lib", __FILE__)
 require "arbre/version"

spec/arbre/unit/context_spec.rb

@@ -1,4 +1,3 @@
-# coding: utf-8
 # frozen_string_literal: true
 require 'spec_helper'