Repositories: Remove default repository URL constants.

Since automatic updates are enabled by default, these will already
be in the system repository settings.

build: Cleanup and fixes following previous commits.

* Hashing semantics for the new build repositories are different than
the old ones, so update those (if the x86 build was not broken before
it is now...)
* OptionalPackages has been updated slightly (removed libtool and git_cvs
from the default images, as they are rarely used nowadays and would pull
in a bunch of dependencies we don't really care for either)
* Removed lib:libqrencode from Haiku package requires (qrencode_kdl is a
static library, the userland libqrencode is not used anywhere in the tree,
as far as I can tell)
* Fix build of JPEG2000 translator after update
* Decouple fluidsynth build machinery and remove from image now that it
is no longer used
* Update repository URL in Repositories preflet

constants.h: fix gcc6 build.

Signed-off-by: Adrien Destugues <pulkomandy@pulkomandy.tk>

Ticket : #13291

Add the new Repositories preflet.

Also includes changes to HaikuDepot to change wording and add the menu item
to open the Repositories preflet.

Signed-off-by: Augustin Cavalier <waddlesplash@gmail.com>
Closes #13147.