[Fix rubocop#4168] Removed shorthand no-color option -n

This option was broken in a5ce6d and the community decided to remove it. Here's why:
* Nobody used it. It has been broken for over a year before it was noticed it's broken
* `-n` is not the most intuitive option for disabling color
* There is no plain way to fix it

CHANGELOG.md

@@ -5,6 +5,7 @@
 ### Changes
 
 * [#4219](https://github.com/bbatsov/rubocop/issues/4219): Add a link to style guide for `Style/IndentationConsistency` cop. ([@pocke][])
+* [#4168](https://github.com/bbatsov/rubocop/issues/4168): Removed `-n` option. ([@sadovnik][])
 
 ### Bug fixes
 
@@ -2716,3 +2717,4 @@
 [@betesh]: https://github.com/betesh
 [@dpostorivo]: https://github.com/dpostorivo
 [@konto-andrzeja]: https://github.com/konto-andrzeja
+[@sadovnik]: https://github.com/sadovnik

lib/rubocop/options.rb

@@ -142,7 +142,7 @@ def add_boolean_flags(opts) # rubocop:disable Metrics/MethodLength
       end
       option(opts, '-a', '--auto-correct')
 
-      option(opts, '-n', '--[no-]color') { |c| @options[:color] = c }
+      option(opts, '--[no-]color') { |c| @options[:color] = c }
 
       option(opts, '-v', '--version')
       option(opts, '-V', '--verbose-version')

spec/rubocop/options_spec.rb

@@ -90,7 +90,7 @@ def abs(path)
               -R, --rails                      Run extra Rails cops.
               -l, --lint                       Run only lint cops.
               -a, --auto-correct               Auto-correct offenses.
-              -n, --[no-]color                 Force color output on or off.
+                  --[no-]color                 Force color output on or off.
               -v, --version                    Display version.
               -V, --verbose-version            Display verbose version.
               -s, --stdin                      Pipe source from STDIN.