We revert commit "Create CMake Policy CMP0015 to fix set(CACHE)" because
the NEW behavior of the policy breaks a valid use case:

  # CMakeLists.txt
  option(BUILD_SHARED_LIBS "..." ON)
  add_library(mylib ...)
  set(BUILD_SHARED_LIBS OFF) # we want only mylib to be shared
  add_subdirectory(ThirdParty)

  # ThirdParty/CMakeLists.txt
  option(BUILD_SHARED_LIBS "..." ON)
  # uh, oh, with NEW behavior this dir uses shared libs!!!

We'll re-introduce the policy later with a different change in behavior
to resolve the motivating case, which was more subtle but less common.

See issue #9008.