The fact that it was still there on the classpath means it needs to
be configured (and fails silently if you don't). Removed to give
us some tmie to reconsider.

It's better maintained and it seems to provide more features
as well.

Vet, Owner, Visit. The Visit "aggregate" is a little artificial
but it demonstrates a useful point about not holding on to
references of "parent" (reference data) objects, i.e. the Visit has
an Integer petId, instead of a Pet field. In principle this app is
now almost ready to migrate to multiple services if anyone wanted
to do that.

Upgrade to Ehcache 3 and Java config

I saw on twitter the reference of an article (in french):
http://javaetmoi.com/2016/08/migrer-vers-spring-boot/

That article concludes with something along the lines of  "Besides the
EhCache and Maven configuration, Petclinic does not hold a single line
of XML anymore".

Looking at the code, we can remove more XML if you want. This PR migrates
the cache infrastructure to EhCache 3 and JCache (JSR-107). This also
reduces the number of dependencies.