core/download: drop the SSH command

core/download: drop the SSH command

The ssh command was added back in 2011 with commit c61788f09 (GENTARGETS:
add support for scp://) and was used to check that the remote file
existed, back when we supported 'make source-check'.

However, in 2017, with commit bf28a165d (pkg-{download, generic}: remove
source-check), we actually removed support for source-check.

The SSH command however was not removed then, and stuck, even though
nothing ever uses it It is not even exported in the environment, and scp
does not use it either (it has -S to specify an ssh-compatible program).

Get rid of it.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>