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 9474cc25.

[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>