slides/buildroot-basic: fixup Buildroot fetching instructions

Since commit 8afe415, Michael has
removed cloned using git:// and replaced by cloning from https://. But
this has caused the text to be inconsistent: we suggest to use
https://, and if it doesn't work because it's blocked by a firewall,
we suggest to use ... https://.

Fixup that, and explain only https://.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

labs/buildroot-basic/buildroot-basic.tex

@@ -34,19 +34,6 @@ \section{Download Buildroot}
 git clone https://git.buildroot.net/buildroot
 \end{bashinput}
 
-If your company blocks the {\em Git} protocol (they should not!), then
-you can try to clone over HTTP:
-
-\begin{bashinput}
-git clone https://git.buildroot.net/git/buildroot
-\end{bashinput}
-
-In the worst case, if neither work, you can download the Buildroot
-tarball \code{buildroot-2021.02.tar.bz2} from
-\code{https://buildroot.org/downloads/} and extract it. However in this
-case, you won't be able to use {\em Git} to visualize your changes and
-keep track of them.
-
 Go into the newly created \code{buildroot} directory.
 
 We're going to start a branch from the {\em 2021.02} Buildroot