Issue/remove continuations plugin support

Removed special support for Scala continuations

The Scala IDE used to package the continuations plug-in to match the Scala
distribution.  While this conveniency was appreciated by the few using
continuations in their projects, it imposed a technical debt on the Scala IDE
codebase (have a look at how the compiler `Settings` used to be instantiated).

Recently, the continuations plug-in has been refactored and splitted into two
separate JARs (a compiler plug-in, and a library). After this modularization,
the Scala IDE codebase could no longer be compiled inside Eclipse because the
continuations library isn't included in the project's classpath. Of course, we
could implement a workaround to restore the functionality, but it just doesn't
seem worth the time, considering the Scala Team has deprecated the
continuations plug-in and will effectively drop supporting it the moment 2.12
is released.  Hence, the decision of dropping the out-of-the-box support in the
Scala IDE.

From now on, if you want to use continutions in a project, you will have to
provide the location of the continuations JAR via the -Xplugins setting.

This commit undoes all changes related to supporting continuations made by
@adriaanm in scala-ide#604

Fix #1002012
Fix #1002011