Clean up monkey patches

* delete `#remove` method
* remove `#delete_prefix` spec, it was only used for Ruby < 2.5 (see 86cfa2f5cf)
* add specifications for `#colorize` and `#uniq`

lib/colorls/monkeys.rb

@@ -5,10 +5,6 @@ class String
     self.color(color)
   end
 
-  def remove(pattern)
-    gsub(pattern, '')
-  end
-
   def uniq
     split('').uniq.join
   end

spec/color_ls/monkey_spec.rb

@@ -1,23 +1,13 @@
+require 'colorls/monkeys'
 
-RSpec.describe "String#delete_prefix" do
-  # back compat for Ruby < 2.5
-
-  it "returns a copy of the string, with the given prefix removed" do
-    expect('hello'.delete_prefix('hell')).to eq 'o'
-    expect('hello'.delete_prefix('hello')).to eq ''
-  end
-
-  it "returns a copy of the string, when the prefix isn't found" do
-    s = 'hello'
-    r = s.delete_prefix('hello!')
-    expect(r).not_to equal s
-    expect(r).to be == s
-    r = s.delete_prefix('ell')
-    expect(r).not_to equal s
-    expect(r).to be == s
-    r = s.delete_prefix('')
-    expect(r).not_to equal s
-    expect(r).to be == s
-  end
-
+RSpec.describe String, '#uniq' do
+ it 'removes all duplicate characters' do
+   expect('abca'.uniq).to be == 'abc'
+ end
+end
+
+RSpec.describe String, '#colorize' do
+  it 'colors a string with red' do
+    expect('hello'.colorize(:red)).to be == Rainbow('hello').red
+  end
 end