3rdparty: Remove chromeec submodule

The chromeec submodule is the largest submodule being pulled into the
coreboot tree, at over 400MB. The main branch also contains the majority
of these commits, so restricting it to a single branch still fetches
over 350MB.

Because there is only a single mainboard directory that enables the
build of the chromeec codebase by default, most people are fetching this
repo for no reason.

Based on this, we're going to change the way that the chromeec submodule
is used, fetching it the way we currently fetch external payloads. This
gives us 2 large advantages:
1) Only builds that actually need the chromeec repo will pull it down.
2) Each board that wants to build the chromeec codebase can use a
different commit, unlike submodules which all use the same "current"
commit.

Signed-off-by: Martin Roth <gaumless@gmail.com>
Change-Id: I357c4c9b506dd3817a308232446144ae889bc220
Reviewed-on: https://review.coreboot.org/c/coreboot/+/81024
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Matt DeVillier <matt.devillier@gmail.com>
Reviewed-by: Julius Werner <jwerner@chromium.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>

.gitmodules

@@ -13,9 +13,6 @@
 [submodule "arm-trusted-firmware"]
 	path = 3rdparty/arm-trusted-firmware
 	url = https://review.coreboot.org/arm-trusted-firmware.git
-[submodule "3rdparty/chromeec"]
-	path = 3rdparty/chromeec
-	url = https://review.coreboot.org/chrome-ec.git
 [submodule "libhwbase"]
 	path = 3rdparty/libhwbase
 	url = https://review.coreboot.org/libhwbase.git