Do not rely on environment changes (like exporting REG_HALT_UNREGISTER)
to persist between different hooks run. This helps when the hooks are
run in different new environments every time.
Instead, in 99-unregister redo the same checks on REG_METHOD as done in
00-rhel-registration, still respecting REG_HALT_UNREGISTER in case the
user does not want to unregister the image generated.
Change-Id: Id594dcd72334f38a2fa96da21206da77a83d7a1a
Closes-Bug: #1434431
8d19032a4a