btrfs-progs: docs: remove EXTLINUX from bootloaders

There has been no update since 2014 and it's likely broken due to many
new features added to btrfs since then.

Signed-off-by: David Sterba <[email protected]>

Author: kdave

Documentation/ch-bootloaders.rst

@@ -4,10 +4,6 @@ booting from BTRFS with respect to features.
 U-boot (https://www.denx.de/wiki/U-Boot/) has decent support for booting but
 not all BTRFS features are implemented, check the documentation.
 
-EXTLINUX (from the https://syslinux.org project) has limited support for BTRFS
-boot and hasn't been updated for for a long time so is not recommended as
-bootloader.
-
 In general, the first 1MiB on each device is unused with the exception of
 primary superblock that is on the offset 64KiB and spans 4KiB. The rest can be
 freely used by bootloaders or for other system information. Note that booting