Revert "Ensure rspec core is loaded if rspec-rails is." (#1678)

This reverts commit 994cb7a.

The original fix was never shown to have fixed anything, and looks
to have triggered #1645 and rspec/rspec-core#2301.

Fixes #1645.

Author: myronmarston

Changelog.md

@@ -1,7 +1,15 @@
+### Development
+[Full Changelog](http://github.com/rspec/rspec-rails/compare/v3.5.1...master)
+
+Bug Fixes:
+
+* Stop unnecessarily loading `rspec/core` from `rspec/rails` to avoid
+  IRB context warning. (Myron Marston, #1678)
+
 ### 3.5.1 / 2016-07-08
 [Full Changelog](http://github.com/rspec/rspec-rails/compare/v3.5.0...v3.5.1)
 
-Bugfixes:
+Bug Fixes:
 
 * Only attempt to load `ActionDispatch::IntegrationTest::Behavior` on Rails 5,
   and above; Prevents possible `TypeError` when an existing `Behaviour` class

lib/rspec-rails.rb

@@ -1,5 +1,3 @@
-# Explicitly require rspec-core for weird spring issue, see #1558
-require 'rspec/core'
 require 'rspec/rails/feature_check'
 
 # Namespace for all core RSpec projects.