mbedtls/external: remove broken git submodule

mbedtls/external: remove broken git submodule

When we squash imported mbedtls, the git submodule "framework" was
preserved in the commit. However, U-Boot itself does not use git
submodule, and provides no .gitmodules file to specify the submodule
repository.

This is normally not an issue when cloning U-Boot repository. However,
when U-Boot is imported as a submodule, this will break git option
`--recurse-submodules` as it fails to resolve "framework".

As we do not use the submodule, remove it to unbreak existing workflows.

Fixes: 12f1212e95fe ("Merge commit '0344c602eadc0802776b65ff90f0a02c856cf53c' as 'lib/mbedtls/external/mbedtls'")
Signed-off-by: ZHANG Yuntian <yt@radxa.com>