dont import main context in download context

By importing the spring main context (intended to be shared
between wicket and other servlets) in the context for the
download servlet, <context:component-scan> is imported,
which finds components within a given package, and does
autowiring on them through an implied <context:annotation-config>.

However, the download-context is used for the download-servlet
by the dispatcher-servlet with the main-context as parent-context.

Importing the main-context in the download-context means the
component-scan is done twice.  This raises a warning that the
TapPU persistence unit name is already used, because the
org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean
is instantiated twice.

For this reason, declare the download-controller bean explicitly
with just a <context:annotation-config> to get the autowiring done.

An alternative would be to use <context:component-scan>, but
on a package that does not include the services detected by
the main config.