Remove third party submodules

Instead use CMake's FetchContent feature.

Git submodules have many well-documented pain points.

A similar change has previously been attempted but was reverted by
revision 2d87391f because FetchContent's
git-clone was found to occasionally fail for no discernable reason.
The difference with this change is that it downloads source archives
instead of cloning Git repos. This is believed to cause less load and
therefore reduce the likelihood of getting rate-limited.
Should failures start to occur again then the sources should be
pre-downloaded and FETCHCONTENT_SOURCE_DIR_<uppercaseName> CMake
variables should be used to point at them.

Signed-off-by: Michael Platings <michael.platings@arm.com>
Change-Id: I12873b540b3fccece6e1e6bb885d574a586f0307