Returns a copy of +self+ with leading substring <tt>prefix</tt> removed:

  'hello'.delete_prefix('hel')      # => "lo"
  'hello'.delete_prefix('llo')      # => "hello"
  'тест'.delete_prefix('те')        # => "ст"
  'こんにちは'.delete_prefix('こん')  # => "にちは"

Related: String#delete_prefix!, String#delete_suffix.