Remove dirs and files across mounts in containers

When running in a container, the root filesystem uses OverlayFS. Due to
a quirk of the OverlayFS implementation, nftw() will not include certain
files in the walk since their device number differs from the parent
directory. We remove the FTW_MOUNT flag to ensure that all child file and
directories are deleted before deleting the parent.

Bug: 465804122

Author: Brian Daniels

base/cvd/cuttlefish/common/libs/utils/files.cpp

@@ -330,7 +330,7 @@ Result<void> RecursivelyRemoveDirectory(const std::string& path) {
     return 0;
   };
 
-  if (nftw(path.c_str(), callback, 128, FTW_DEPTH | FTW_MOUNT | FTW_PHYS) < 0) {
+  if (nftw(path.c_str(), callback, 128, FTW_DEPTH | FTW_PHYS) < 0) {
     return CF_ERRNO("Failed to remove directory \""
                     << path << "\": " << strerror(errno));
   }