rmuf: Don't delete writes to external fields
rmuf: Don't delete writes to external fields

Summary:
Apparently can_delete / can_rename will return true for external fields...
seems like we should fix that API sometime...

Reviewed By: kgsharma, emma0303

Differential Revision: D7387576

fbshipit-source-id: 9f9d724248814617f4a86a20a769b1674cfbeb92

facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex