build: only run findbugs on a project when it has java source files

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>
build: remove no.{junit,findbugs} properties

junit and findbugs targets only run when needed, depending on whether
the test resp src folders contain java files

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>