Remove yard-doctest autoloading (googleapis#2133)

This was adding the plugin multiple times, and would print the following
warning multiple times when a rake task was run:

[error]: Error loading plugin 'yard-doctest'

google-cloud-bigquery/Rakefile

@@ -134,7 +134,6 @@ end
 
 desc "Run yard-doctest example tests."
 task doctest: :yard do
-  sh "bundle exec yard config -a autoload_plugins yard-doctest"
   sh "bundle exec yard doctest"
 end
 

google-cloud-datastore/Rakefile

@@ -81,7 +81,6 @@ end
 
 desc "Run yard-doctest example tests."
 task doctest: :yard do
-  sh "bundle exec yard config -a autoload_plugins yard-doctest"
   sh "bundle exec yard doctest"
 end
 

google-cloud-debugger/Rakefile

@@ -131,7 +131,6 @@ end
 
 desc "Run yard-doctest example tests."
 task doctest: :yard do
-  sh "bundle exec yard config -a autoload_plugins yard-doctest"
   sh "bundle exec yard doctest"
 end
 

google-cloud-dns/Rakefile

@@ -109,7 +109,6 @@ end
 
 desc "Run yard-doctest example tests."
 task doctest: :yard do
-  sh "bundle exec yard config -a autoload_plugins yard-doctest"
   sh "bundle exec yard doctest"
 end
 

google-cloud-error_reporting/Rakefile

@@ -85,7 +85,6 @@ end
 
 desc "Run yard-doctest example tests."
 task doctest: :yard do
-  sh "bundle exec yard config -a autoload_plugins yard-doctest"
   sh "bundle exec yard doctest"
 end
 

google-cloud-firestore/Rakefile

@@ -187,7 +187,6 @@ end
 
 desc "Run yard-doctest example tests."
 task doctest: :yard do
-  sh "bundle exec yard config -a autoload_plugins yard-doctest"
   sh "bundle exec yard doctest"
 end
 

google-cloud-logging/Rakefile

@@ -113,7 +113,6 @@ end
 
 desc "Run yard-doctest example tests."
 task doctest: :yard do
-  sh "bundle exec yard config -a autoload_plugins yard-doctest"
   sh "bundle exec yard doctest"
 end
 

google-cloud-pubsub/Rakefile

@@ -108,7 +108,6 @@ end
 desc "Run yard-doctest example tests."
 #task doctest: :yard do
 task :doctest do
-  sh "bundle exec yard config -a autoload_plugins yard-doctest"
   sh "bundle exec yard doctest"
 end
 

google-cloud-resource_manager/Rakefile

@@ -41,7 +41,6 @@ end
 
 desc "Run yard-doctest example tests."
 task doctest: :yard do
-  sh "bundle exec yard config -a autoload_plugins yard-doctest"
   sh "bundle exec yard doctest"
 end
 

google-cloud-spanner/Rakefile

@@ -114,7 +114,6 @@ end
 
 desc "Run yard-doctest example tests."
 task :doctest do
-  sh "bundle exec yard config -a autoload_plugins yard-doctest"
   sh "bundle exec yard doctest"
 end
 

google-cloud-storage/Rakefile

@@ -121,7 +121,6 @@ end
 
 desc "Run yard-doctest example tests."
 task doctest: :yard do
-  sh "bundle exec yard config -a autoload_plugins yard-doctest"
   sh "bundle exec yard doctest"
 end
 

google-cloud-trace/Rakefile

@@ -114,7 +114,6 @@ end
 
 desc "Runs yard-doctest example tests."
 task :doctest do
-  sh "bundle exec yard config -a autoload_plugins yard-doctest"
   sh "bundle exec yard doctest"
 end
 

google-cloud-translate/Rakefile

@@ -105,7 +105,6 @@ end
 
 desc "Run yard-doctest example tests."
 task doctest: :yard do
-  sh "bundle exec yard config -a autoload_plugins yard-doctest"
   sh "bundle exec yard doctest"
 end
 

google-cloud-vision/Rakefile

@@ -81,7 +81,6 @@ end
 
 desc "Run yard-doctest example tests."
 task doctest: :yard do
-  sh "bundle exec yard config -a autoload_plugins yard-doctest"
   sh "bundle exec yard doctest"
 end