RFR (trivial) 8205327: Clean up #if INCLUDE_CDS in classLoaderExt.cpp and classLoaderExt.hpp

[Jiangli Zhou]

Please review this trivial change that cleans up #if INCLUDE_CDS in 
classLoaderExt.cpp and classLoaderExt.hpp.

- Removed #if INCLUDE_CDS from classLoaderExt.cpp since the file is not 
included when CDS feature is excluded.
- Cleaned up classLoaderExt.hpp and moved CDS_ONLY functions & enum 
under #if INCLUDE_CDS.

webrev: http://cr.openjdk.java.net/~jiangli/8205327/webrev.00/
RFE: https://bugs.openjdk.java.net/browse/JDK-8205327

Tested with normal and minimum builds locally on linux-x64. Running 
tier1-tier5 tests.

Thanks,
Jiangli