Remove Spring Data Solr support

In preparation to the upgrade to Spring Data 2021.0, this commit removes
auto-configuration for Spring Data Solr.

Closes gh-24939

spring-boot-project/spring-boot-starters/spring-boot-starter-data-solr/build.gradle

@@ -1,13 +0,0 @@
-plugins {
-	id "org.springframework.boot.starter"
-}
-
-description = "Starter for using the Apache Solr search platform with Spring Data Solr"
-
-dependencies {
-	api(project(":spring-boot-project:spring-boot-starters:spring-boot-starter"))
-	api("org.springframework.data:spring-data-solr") {
-		exclude group: "commons-logging", module: "commons-logging"
-		exclude group: "org.slf4j", module: "jcl-over-slf4j"
-	}
-}