doc/manual: rsync is not optional

rsync is used in the infrastructure, mostly for the per-package infra,
and for the override-srcdir mechanism, but also to build the manual.
As such, it is not optional but mandatory, and already listed so.

Drop the reference to rsync from the list of optional packages.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

docs/manual/prerequisite.adoc

@@ -69,7 +69,7 @@ development packages typically have a _-dev_ or _-devel_ suffix.
 In the official tree, most of the package sources are retrieved using
 +wget+ from _ftp_, _http_ or _https_ locations. A few packages are only
 available through a version control system. Moreover, Buildroot is
-capable of downloading sources via other tools, like +rsync+ or +scp+
+capable of downloading sources via other tools, like +git+ or +scp+
 (refer to xref:download-infra[] for more details). If you enable
 packages using any of these methods, you will need to install the
 corresponding tool on the host system:
@@ -78,7 +78,6 @@ corresponding tool on the host system:
 ** +cvs+
 ** +git+
 ** +mercurial+
-** +rsync+
 ** +scp+
 ** +sftp+
 ** +subversion+