chore(diagnostics): don't pass .rubocop.yml file path to RuboCop

RuboCop will itself find and use any relevant configuration files based
on the file path of the file it is analyzing. Hence there is no need for
Solargraph to have it's own implementation of this logic.

lib/solargraph/diagnostics/rubocop_helpers.rb

@@ -13,32 +13,13 @@ module RubocopHelpers
       # @param code [String]
       # @return [Array(Array<String>, Array<String>)]
       def generate_options filename, code
-        args = ['-f', 'j']
-        rubocop_file = find_rubocop_file(filename)
-        args.push('-c', fix_drive_letter(rubocop_file)) unless rubocop_file.nil?
-        args.push filename
+        args = ['-f', 'j', filename]
         base_options = RuboCop::Options.new
         options, paths = base_options.parse(args)
         options[:stdin] = code
         [options, paths]
       end
 
-      # Find a RuboCop configuration file in a file's directory tree.
-      #
-      # @param filename [String]
-      # @return [String, nil]
-      def find_rubocop_file filename
-        return nil unless File.exist?(filename)
-        filename = File.realpath(filename)
-        dir = File.dirname(filename)
-        until File.dirname(dir) == dir
-          here = File.join(dir, '.rubocop.yml')
-          return here if File.exist?(here)
-          dir = File.dirname(dir)
-        end
-        nil
-      end
-
       # RuboCop internally uses capitalized drive letters for Windows paths,
       # so we need to convert the paths provided to the command.
       #

spec/diagnostics/rubocop_helpers_spec.rb

@@ -1,11 +1,4 @@
 describe Solargraph::Diagnostics::RubocopHelpers do
-  it "finds a .rubocop.yml file in a parent directory" do
-    file = File.realpath(File.join 'spec', 'fixtures', 'rubocop-subfolder-configuration', 'folder1', 'folder2', 'test.rb')
-    conf = File.realpath(File.join 'spec', 'fixtures', 'rubocop-subfolder-configuration', '.rubocop.yml')
-    found = Solargraph::Diagnostics::RubocopHelpers.find_rubocop_file(file)
-    expect(found).to eq(conf)
-  end
-
   it "converts lower-case drive letters to upper-case" do
     input = 'c:/one/two'
     output = Solargraph::Diagnostics::RubocopHelpers.fix_drive_letter(input)