Plugins: Revert 6.5.3 changes for showing an admin notice on successful activation.

[hellofromTonya]

With a new direction in the 6.5.4 minor to restore the Activate button's href (see #61319 / [58250]), the changesets committed for 6.5.3 (see #60992) are now dead code and will not be rendered.

After discussing in ​Make/Core slack, consensus is to revert both [58081] (on trunk) and [58083] (on 6.5 branch).

​Quoting @costdev:

Sure, 68081 (trunk) and 68083 (6.5) should be reverted. They are part of a solution that did not achieve the intended result fully, and have been replaced, not extended, with a different solution for 6.5. If we want to introduce them later, then we still have the code stored in history to save us some time.

As @jorbin ​noted:

I'm not sure if it will be a clean revert since [58250] also changed some of these lines, but I would support reverting the functionality

[hellofromTonya]

In reviewing and testing the revert of [58081] on the current trunk, the revert is clean. Pushing the revert commit shortly.

[hellofromTonya]

Clean revert diff of r58081

[hellofromTonya]

In 58257:

Plugins: Revert r58081 show an admin notice on successful activation.

With a new direction in the 6.5.4 to restore the Activate button's href (see [58250]), the changesets committed for 6.5.3 (see #60992) are now dead code and will not render the admin notice on successful plugin activation.

This commit is a clean revert of r58081.

Follow-up to [58250].

Props swissspidy, azaozz, costdev, jorbin, hellofromTonya, afragen.
Fixes #61331.
See #60992, #61319.

[hellofromTonya]

Reopening for 2nd committer sign-off to revert r58083 on the 6.5-branch.

[jorbin]

[58083] looks good for backport to the 6.5 branch.

[hellofromTonya]

In 58258:

Plugins: Revert r58083 show an admin notice on successful activation.

With a new direction in the 6.5.4 to restore the Activate button's href (see #61319 / [58250]), the changesets committed for 6.5.3 (see #60992) are now dead code and will not render the admin notice on successful plugin activation.

This commit is a clean revert of r58083.

Follow-up to [58250], [58257].

Reviewed by jorbin.
Reverts [58083] on the 6.5 branch.

Props swissspidy, azaozz, costdev, jorbin, hellofromTonya, afragen.
Fixes #61331.
See #60992.