documentation: remove broken link to gmane (received_uuid patch in kernel 4.4)

gmane is dead, see kernel patch:

b96b1db039 btrfs: fix resending received snapshot with parent

doc/FAQ.md

@@ -205,12 +205,6 @@ effect, this also detects possible read-errors on your backup targets
 
 See **btrbk archive** command in [btrbk(1)] for more details.
 
-**Note that kernels >=4.1 and <4.4 have a bug when re-sending
-subvolumes**, make sure you run a recent/patched kernel or step 3 will
-fail. Read
-[this thread on gmane](http://thread.gmane.org/gmane.comp.file-systems.btrfs/48798)
-(the patch provided is confirmed working on kernels 4.2.x and 4.3.x).
-
 
 ### Answer 2: Use external storage as "stream-fifo"
 

doc/btrbk.1.asciidoc

@@ -227,16 +227,11 @@ retention policy).
     continue using btrbk after swapping your backup disk with the
     archive disk.
 +
-Note that this feature needs a *linux kernel >=4.4* to work correctly!
+If you want to use nested subvolumes on the target filesystem, you
-Kernels >=4.1 and <4.4 have a bug when re-sending subvolumes (the
+need to create them by hand (e.g. by running "btrfs subvolume create
-archived subvolumes will have incorrect received_uuid, see
+<target>/dir"). Check the output of --dry-run if unsure.
-<http://thread.gmane.org/gmane.comp.file-systems.btrfs/48798>), so
-make sure you run a recent kernel.
 +
-Known bugs: If you want to use nested subvolumes on the target
+Note that this feature needs a *linux kernel >=4.4* to work correctly!
-filesystem, you need to create them by hand (e.g. by running "btrfs
-subvolume create <target>/dir"). Check the output of --dry-run if
-unsure.
 
 *clean* [filter...]::
     Delete incomplete (garbled) backups. Incomplete backups can be