Moves responsibility to import SamplerAutoConfiguration to core

Before, spring-cloud-sleuth-zipkin had to import `SamplerAutoConfiguration`
directly to unwind a sampler ordering problem caused by `TraceAutoConfiguration`
defining the default `Sampler` bean.

This fixes that by moving the default `Sampler` to where it belongs
(`SamplerAutoConfiguration`) and having `TraceAutoConfiguration` import
the sampling configuration directly as opposed to relying on auto-configuration
ordering. Finally it removes the mistake of setting `SamplerAutoConfiguration`
as auto-configuration in the first place.

The name `SamplerAutoConfiguration` was left alone because changing it would
interfere with 3rd party code that formerly imported it to correct this issue
in their non-zipkin setups.

Fixes #1618

spring-cloud-sleuth-zipkin/src/main/java/org/springframework/cloud/sleuth/zipkin2/ZipkinAutoConfiguration.java

@@ -36,7 +36,6 @@ import org.springframework.boot.context.properties.EnableConfigurationProperties
 import org.springframework.cloud.client.serviceregistry.Registration;
 import org.springframework.cloud.commons.util.InetUtils;
 import org.springframework.cloud.sleuth.autoconfig.TraceAutoConfiguration;
-import org.springframework.cloud.sleuth.sampler.SamplerAutoConfiguration;
 import org.springframework.cloud.sleuth.zipkin2.sender.ZipkinSenderConfigurationImportSelector;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
@@ -46,8 +45,7 @@ import org.springframework.web.client.RestTemplate;
 
 /**
  * {@link org.springframework.boot.autoconfigure.EnableAutoConfiguration
- * Auto-configuration} enables reporting to Zipkin via HTTP. Has a default sampler set
- * from the {@link SamplerAutoConfiguration}
+ * Auto-configuration} enables reporting to Zipkin via HTTP.
  *
  * The {@link ZipkinRestTemplateCustomizer} allows you to customize the
  * {@link RestTemplate} that is used to send Spans to Zipkin. Its default implementation -
@@ -56,7 +54,6 @@ import org.springframework.web.client.RestTemplate;
  * @author Spencer Gibb
  * @author Tim Ysewyn
  * @since 1.0.0
- * @see SamplerAutoConfiguration
  * @see ZipkinRestTemplateCustomizer
  * @see DefaultZipkinRestTemplateCustomizer
  */
@@ -66,7 +63,7 @@ import org.springframework.web.client.RestTemplate;
 		"spring.zipkin.enabled" }, matchIfMissing = true)
 @AutoConfigureBefore(TraceAutoConfiguration.class)
 @AutoConfigureAfter(name = "org.springframework.cloud.autoconfigure.RefreshAutoConfiguration")
-@Import({ ZipkinSenderConfigurationImportSelector.class, SamplerAutoConfiguration.class })
+@Import(ZipkinSenderConfigurationImportSelector.class)
 public class ZipkinAutoConfiguration {
 
 	/**