Cleanup LICENSE file.

Remove copyright attributions to files that no longer exist; Change the
year to 2008 to be consistent with the copyright header of other source
files.

LICENSE

-This license applies to all parts of Protocol Buffers except the following:
-
-  - Atomicops support for generic gcc, located in
-    src/google/protobuf/stubs/atomicops_internals_generic_gcc.h.
-    This file is copyrighted by Red Hat Inc.
-
-  - Atomicops support for AIX/POWER, located in
-    src/google/protobuf/stubs/atomicops_internals_power.h.
-    This file is copyrighted by Bloomberg Finance LP.
-
-Copyright 2014, Google Inc.  All rights reserved.
+Copyright 2008 Google Inc.  All rights reserved.
 
 Redistribution and use in source and binary forms, with or without
 modification, are permitted provided that the following conditions are