Remove jetty-jsp from Spring Boot Starter Jetty

The documentation claims that JSPs don't work with embedded Jetty
making the dependency on jetty-jsp pointless. Furthermore,
spring-boot-starter-tomcat doesn't include JSP support and Undertow
doesn't support JSPs at all so removing jetty-jsp makes the embedded
container starters more consistent. It also removes 3.2MB from a
Jetty-based fat jar (spring-boot-sample-jetty drops from 12MB to
8.8MB).

Closes gh-2680

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

@@ -18,10 +18,6 @@
 		<main.basedir>${basedir}/../..</main.basedir>
 	</properties>
 	<dependencies>
-		<dependency>
-			<groupId>org.eclipse.jetty</groupId>
-			<artifactId>jetty-jsp</artifactId>
-		</dependency>
 		<dependency>
 			<groupId>org.eclipse.jetty</groupId>
 			<artifactId>jetty-webapp</artifactId>