Gemfile: drop redcarpet gem

This was used as part of the v1 book build, but we dropped that in
f474e35 (Remove link to 1st book edition, 2018-11-21).

Author: peff

Gemfile

@@ -16,7 +16,6 @@ gem "tilt"
 
 gem "diffy"
 gem "nokogiri"
-gem "redcarpet"
 gem "yajl-ruby"
 
 # Assets

Gemfile.lock

@@ -202,7 +202,6 @@ GEM
     rb-fsevent (0.10.4)
     rb-inotify (0.10.1)
       ffi (~> 1.0)
-    redcarpet (3.5.1)
     redis (4.2.5)
     redis-actionpack (5.2.0)
       actionpack (>= 5, < 7)
@@ -336,7 +335,6 @@ DEPENDENCIES
   rails (~> 6.0)
   rails-controller-testing
   rails_12factor
-  redcarpet
   redis-rails
   rspec-rails
   rubocop-github