Remove 4.8.1 UDFs: it was never released.

wiki/src/upgrade/v2/Tails/4.8.1/amd64/alpha/upgrades.yml

----
-build-target: amd64
-channel: alpha
-initial-install-version: 4.8.1
-product-name: Tails

wiki/src/upgrade/v2/Tails/4.8.1/amd64/alpha/upgrades.yml.pgp

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wiki/src/upgrade/v2/Tails/4.8.1/amd64/stable/upgrades.yml

----
-build-target: amd64
-channel: stable
-initial-install-version: 4.8.1
-product-name: Tails

wiki/src/upgrade/v2/Tails/4.8.1/amd64/stable/upgrades.yml.pgp

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