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