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](https://github.com/rails/rails/blob/e21bd333199888aa76112298735dbb9988a2d72d/activemodel/test/models/sheep.rb) in activemodel/test/models/sheep.rb