Remove unused Sheep fixture from AV

The `Sheep` fixture was added in eb23754e when moving template tests
from actionpack to actionview, but it's not actually used in ActionView tests.

The `Sheep` fixture is only used to test `uncountable` in ActiveModel tests,
and is already defined in activemodel/test/models/sheep.rb
Merge pull request #18129 from claudiob/remove-unused-sheep-fixture-from-action-view

Remove unused Sheep fixture from AV