platform: delete #known_engines altogether

It turns out this was actually never released so there's nothing to deprecate.

lib/pry/helpers/base_helpers.rb

@@ -130,20 +130,6 @@ module Pry::Helpers::BaseHelpers
     Pry::Helpers::Platform.mri_2?
   end
 
-  @known_engines_warn = false
-  # @deprecated This will be removed in the next release.
-  def known_engines
-    unless @known_engines_warn
-      loc = caller_locations(1..1).first
-      warn(
-        "#{loc.path}:#{loc.lineno}: warning: BaseHelpers.#{__method__} is " \
-        "deprecated and will be removed in the next Pry release"
-      )
-      @known_engines_warn = true
-    end
-    [:jruby, :mri]
-  end
-
   def silence_warnings
     old_verbose = $VERBOSE
     $VERBOSE = nil

lib/pry/platform.rb

@@ -84,14 +84,6 @@ class Pry
     def mri_2?
       !!(mri? and RUBY_VERSION =~ /\A2/)
     end
-
-    #
-    #  @return [Array<Symbol>]
-    #    Returns an Array of Ruby engines that Pry is known to run on.
-    #
-    def known_engines
-      [:jruby, :mri]
-    end
   end
 
   # Not supported on MRI 2.2 and lower.