Do not abort when unlinking non-data ruby objects

Fixes issue #1168.
Remove a call to abort() (introduced by commit
0e725b5d9b) made when SWIG_RubyUnlinkObjects()
is called on non T_DATA objects. It can happen when the destruction of T_DATA
objects is deferred: the Ruby GC first turn them to T_ZOMBIE, then calls their
free method (SWIG_RubyUnlinkObjects()).

Examples/ruby/free_function/runme.rb

@@ -39,7 +39,11 @@ GC.start
 # C++ object
 ok = false
 begin
-  puts tiger2.get_name
+  # Let's stress the GC a bit, a single pass might not be enough.
+  10.times {
+    GC.start
+    puts tiger2.get_name
+  }
 rescue ObjectPreviouslyDeleted => error
   ok = true
 end