Revert "testing/tests/download: test git special ref"

This reverts commit 81771cfc.

The download of sha1 of a special ref currently works or not depending
on the git client version in use.
With git version 2.11.0 (present in the docker image) it does not work.
With git version 2.17.1 it works.

For the sake of reproducibility, remove this part of the TestGitRefs
test case until some code gets added to the download infra to handle
sha1 of a special ref for any git client version.

Fixes:
https://gitlab.com/buildroot.org/buildroot/-/jobs/158295269



Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>