From org version 9.5 org will not be distributed from Org ELPA but from
ELPA (see https://list.orgmode.org/87lfa7tc9v.fsf@gnu.org/t/). This commit makes
Spacemacs use/install org from ELPA.
Besides that, this commit removes some 'hack' that seemed to have no effect. As
org comes with Emacs `package-installed-p 'org` will always return t. Also
activating org via `configuration-layer//activate-package` seems to have no
effect.
Finally, the package is declared including a :min-version because appending only
`:location melpa` seems to have no effect (this can probably be considered a
bug).
I am almost certain that the information that was added when making org install
from Org ELPA is still relevant also for installing org from ELPA so I have just
adapted the version number from 0.104 to 0.300.
As the "org" archive has been removed, the `ert-deftest` for it can be removed
also.
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