kernel: Remove unused derived offset symbols

Some of the offset symbols that are derived from the macro
GEN_OFFSET_SYM() are not used anywhere in the Zephyr codebase.
Remove them as part of a cleanup effort.

Instances of an associated GEN_OFFSET_SYM() have also been
removed when the resulting macro is no longer referenced.

Signed-off-by: Peter Mitsis <peter.mitsis@intel.com>