download/git: clarify why .git is removed

download/git: clarify why .git is removed

The removal of the .git dir before creating the tarball is not anymore
just an optimization. It is necessary to make the tarball reproducible.
Also, without the removal, large tarballs (gigabytes) would be created
for some linux trees.

Update the comment accordingly.

Reported-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>