Remove unnecessary compiler configuration

The new `maven.compiler.source` and `maven.compiler.target` properties
have removed the need for explicit configuration.

Closes gh-1684

spring-boot-starters/spring-boot-starter-parent/pom.xml

@@ -60,14 +60,6 @@
 		<pluginManagement>
 			<plugins>
 				<!-- Apply more sensible defaults for user projects -->
-				<plugin>
-					<groupId>org.apache.maven.plugins</groupId>
-					<artifactId>maven-compiler-plugin</artifactId>
-					<configuration>
-						<source>${maven.compiler.source}</source>
-						<target>${maven.compiler.target}</target>
-					</configuration>
-				</plugin>
 				<plugin>
 					<groupId>org.apache.maven.plugins</groupId>
 					<artifactId>maven-jar-plugin</artifactId>