Clean up .classpath files

Some .classpath files still contain references to SCALA_CONTAINER and
SCALA_COMPILER_CONTAINER which are not needed anymore. Furthermore
sdt.core explicitly references JavaSE-1.6 as its runtime environment
which shoudn't be included either because this doesn't allow a JRE
independent build.
Merge pull request #626 from sschaef/t-cleanup-classpath-files

Clean up .classpath files