Removed redundant entries from POM files

Polished POM files to remove redundant entries that were causing warnings in the IDE. Some notable changes are:
- Removed spring-boot.version from sample project as they inherit it from the parent
- Removed versioning for maven-jar-plugin (was 3.0) from multiple project so it relies on the managed version
- Removed java.version from all spring-cloud-function-* modules as they inherit the one from parent.

Addressed PR comments
- removed references to maven-jar-plugin

spring-cloud-function-context/pom.xml

@@ -15,7 +15,6 @@
 	</parent>
 
 	<properties>
-		<java.version>1.8</java.version>
 		<spring-cloud-function.version>1.0.0.BUILD-SNAPSHOT</spring-cloud-function.version>
 	</properties>
 
@@ -35,15 +34,4 @@
 			<optional>true</optional>
 		</dependency>
 	</dependencies>
-
-	<build>
-		<plugins>
-			<plugin>
-				<groupId>org.apache.maven.plugins</groupId>
-				<artifactId>maven-jar-plugin</artifactId>
-				<version>3.0.0</version>
-			</plugin>
-		</plugins>
-	</build>
-
 </project>