Remove ancient 64-btrfs.rules so systemd provided rule is used

This solves the failure to boot when the root fs is on a btrfs handled
raid set-up. The reason it fails is typically because one or both disks
failure are not initialised before the kernel wants to mount root.

This generally affects only disks that are mounted during boot.

Solves issues #1329 and #1980

conf/64-btrfs.rules

@@ -1 +1,17 @@
-RUN+="/bin/mknod /dev/btrfs-control c 10 234"
+# do not edit this file, it will be overwritten on update
+
+SUBSYSTEM!="block", GOTO="btrfs_end"
+ACTION=="remove", GOTO="btrfs_end"
+ENV{ID_FS_TYPE}!="btrfs", GOTO="btrfs_end"
+ENV{SYSTEMD_READY}=="0", GOTO="btrfs_end"
+
+# let the kernel know about this btrfs filesystem, and check if it is complete
+IMPORT{builtin}="btrfs ready $devnode"
+
+# mark the device as not ready to be used by the system
+ENV{ID_BTRFS_READY}=="0", ENV{SYSTEMD_READY}="0"
+
+# reconsider pending devices in case when multidevice volume awaits
+ENV{ID_BTRFS_READY}=="1", RUN+="@rootbindir@/udevadm trigger -s block -p ID_BTRFS_READY=0"
+
+LABEL="btrfs_end"