platformvm: delete dead weak SharedMemory interface declarations

Both executor backends re-declared a local SharedMemory interface with the
weak Apply(map[ids.ID]interface{}, ...interface{}) signature — but nothing
referenced it. The real atomic path uses Runtime.SharedMemory (= the narrow
atomic.SharedMemory: Apply(map[ids.ID]*atomic.Requests, ...database.Batch)).
Pure dead code; removed both. 28 platformvm packages green.

Co-authored-by: Hanzo Dev <dev@hanzo.ai>

vms/platformvm/block/executor/backend.go

@@ -39,12 +39,6 @@ type backend struct {
 	rt *runtime.Runtime
 }
 
-// SharedMemory provides cross-chain atomic operations
-type SharedMemory interface {
-	Get(peerChainID ids.ID, keys [][]byte) ([][]byte, error)
-	Apply(requests map[ids.ID]interface{}, batch ...interface{}) error
-}
-
 func (b *backend) GetState(blkID ids.ID) (state.Chain, bool) {
 	b.blkIDToStateLock.RLock()
 	defer b.blkIDToStateLock.RUnlock()

vms/platformvm/txs/executor/backend.go

@@ -6,7 +6,6 @@ package executor
 import (
 	"github.com/luxfi/runtime"
 	"github.com/luxfi/validators/uptime"
-	"github.com/luxfi/ids"
 	"github.com/luxfi/log"
 	"github.com/luxfi/node/vms/platformvm/config"
 	"github.com/luxfi/node/vms/platformvm/reward"
@@ -27,9 +26,3 @@ type Backend struct {
 	Bootstrapped *utils.Atomic[bool]
 	Log          log.Logger
 }
-
-// SharedMemory provides cross-chain atomic operations
-type SharedMemory interface {
-	Get(peerChainID ids.ID, keys [][]byte) ([][]byte, error)
-	Apply(requests map[ids.ID]interface{}, batch ...interface{}) error
-}