The scenario for which --with-git= was introduced was to use a SOCKS proxy
such as tsocks.  However, this was back in 2017 when QEMU's submodules
used the git:// protocol, and it is not as important when using the
"smart HTTP" backend; for example, neither "meson subprojects download"
nor scripts/checkpatch.pl obey the GIT environment variable.

So remove the knob, but test for the presence of git in the configure and
git-submodule.sh scripts, and suggest using --with-git-submodules=validate
+ a manual invocation of git-submodule.sh when git does not work.  Hopefully
in the future the GIT environment variable will be supported by Meson.

Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>