Don't store the target CL device in the kernel object

doc/reference.rst

doc/tutorial.rst

examples/hello-loopy.py

loopy/__init__.py

loopy/auto_test.py

loopy/check.py

loopy/codegen/__init__.py

loopy/compiled.py

loopy/kernel/__init__.py

loopy/kernel/creation.py

loopy/preprocess.py

loopy/schedule.py

test/test_dg.py

test/test_linalg.py

test/test_loopy.py

test/test_nbody.py

test/test_sem_reagan.py