Revert "manual: Add notes about GitHub and hashes"

Revert "manual: Add notes about GitHub and hashes"

This note is no longer needed, so remove it.

Tarballs generated by GitHub are always the same, so hashes can be
checked.

This reverts commit 9474cc2594c8643301164500ce6eb62546da2b8f.

[Thomas: fix other remaining references to not using hashes for
github, as noticed by Maxime Hadjinlian.]

Cc: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>