Remove unnecessary SkImageDecoder_libjpeg.cpp

Adding files outside $(LOCAL_PATH) is a bad idea. It generates output files
to an unexpected directory. In this case, it's

out/target/external/skia/src/images/SkImageDecoder_libjpeg.o

It's not in $(PRODUCT_OUT), which is not correct.
Worse, it makes the build system be unable to build different
architectures (arm and x86) simultaneously, since the output files
conflcit.

Seems we can just remove SkImageDecoder_libjpeg.cpp since it's
already in libskia.

Change-Id: I3e7533126ef60fe2fd424fdc777760e73c93489b