Delete DB schema migrations for v11 and earlier (#3761)

## Proposed Changes

Now that the Gnosis merge is scheduled, all users should have upgraded beyond Lighthouse v3.0.0. Accordingly we can delete schema migrations for versions prior to v3.0.0.

## Additional Info

I also deleted the state cache stuff I added in #3714 as it turned out to be useless for the light client proofs due to the one-slot offset.

beacon_node/store/src/hot_cold_store.rs

@@ -386,16 +386,6 @@ impl<E: EthSpec, Hot: ItemStore<E>, Cold: ItemStore<E>> HotColdDB<E, Hot, Cold>
         }
     }
 
-    /// Get a schema V8 or earlier full block by reading it and its payload from disk.
-    pub fn get_full_block_prior_to_v9(
-        &self,
-        block_root: &Hash256,
-    ) -> Result<Option<SignedBeaconBlock<E>>, Error> {
-        self.get_block_with(block_root, |bytes| {
-            SignedBeaconBlock::from_ssz_bytes(bytes, &self.spec)
-        })
-    }
-
     /// Convert a blinded block into a full block by loading its execution payload if necessary.
     pub fn make_full_block(
         &self,