Remove frozen string literals from arb templates

It does not have effect there and outputs a warning when running tests
with `RUBYOPTS='-w' rake`

```
warning: 'frozen_string_literal' is ignored after any tokens
```

Behavior tested in `empty.arb` template

```
# frozen_string_literal: true
hello = 'hello'
hello.concat ' world'
```

Expected: fail
Actual: pass

Close #649

.rubocop.yml

@@ -584,6 +584,7 @@ Style/FrozenStringLiteralComment:
   Enabled: true
   Exclude:
     - bin/console
+    - '**/*.arb'
 
 Style/HashSyntax:
   Enabled: true

spec/rails/templates/arbre/_partial.arb

@@ -1,2 +1 @@
-# frozen_string_literal: true
 para "Hello from a partial"

spec/rails/templates/arbre/_partial_with_assignment.arb

@@ -1,2 +1 @@
-# frozen_string_literal: true
 para "Partial: #{my_instance_var}"

spec/rails/templates/arbre/page_with_arb_partial_and_assignment.arb

@@ -1,4 +1,3 @@
-# frozen_string_literal: true
 h1 "Before Partial"
 render "arbre/partial_with_assignment"
 h2 "After Partial"

spec/rails/templates/arbre/page_with_assignment.arb

@@ -1,2 +1 @@
-# frozen_string_literal: true
 h1 my_instance_var

spec/rails/templates/arbre/page_with_erb_partial.arb

@@ -1,4 +1,3 @@
-# frozen_string_literal: true
 h1 "Before Partial"
 render "erb/partial"
 h2 "After Partial"

spec/rails/templates/arbre/page_with_partial.arb

@@ -1,4 +1,3 @@
-# frozen_string_literal: true
 h1 "Before Partial"
 render "arbre/partial"
 h2 "After Partial"

spec/rails/templates/arbre/page_with_render_with_block.arb

@@ -1,4 +1,3 @@
-# frozen_string_literal: true
 render_in_object = Class.new do
   def render_in(_, &block)
     block.call

spec/rails/templates/arbre/simple_page.arb

@@ -1,4 +1,3 @@
-# frozen_string_literal: true
 html do
   head do
   end