Remove outdated buildSrc README

buildSrc/README.adoc

-= BOM Plugin
-
-Allows one to publish a BOM from Gradle.
-Properties, dependencies, and other BOMs are applied to the `bom {}` extension.
-
-This plugin applies the `java-platform` and `maven-publish` plugins to the Project it's applied to.
-
-== Usage
-
-[source,groovy,indent=0]
-----
-plugins {
-	id 'org.springframework.boot.bom'
-}
-
-bom {
-    property 'logback.version', '1.2.3'
-    property 'junit-jupiter.version', '5.3.2'
-
-    dependency 'ch.qos.logback', 'logback-classic', '${logback.version}'
-    dependency 'ch.qos.logback', 'logback-core', '${logback.version}'
-
-    bomImport 'org.junit', 'junit-bom', '${junit-jupiter.version}'
-}
-----
-