Don't require selenium-webdriver

Currently the Gemfile requires `webdrivers` which is all we need for the
current version of the CircleCI orbs.

There's an error we're hoping to resolve by updating this version:
When `selenium-webdriver` and `webdrivers` are included we end up with
this error:

    NoMethodError:
      undefined method `bitsize' for Selenium::WebDriver::Platform:Module

solidus_recently_viewed.gemspec

@@ -35,7 +35,5 @@ Gem::Specification.new do |s|
   s.add_dependency 'solidus_core', ['>= 2.4.0', '< 5']
   s.add_dependency 'solidus_support', '~> 0.8'
 
-
-  s.add_development_dependency 'selenium-webdriver'
   s.add_development_dependency 'solidus_dev_support'
 end