tests: Remove 'override-download' flag from insttest

This flag was previously passed to the 'simple_binary_run.py' to
override downloads. However, since this patch:
https://gem5-review.googlesource.com/c/public/gem5/+/52086
we no longer use the flag. It appears in this patch the insttest test
was not updated and, as such, this test failed:
https://jenkins.gem5.org/job/nightly/38/

This, in part, will fix the nightly build.

Change-Id: I4fb5ab175c73687304c04fe426f81519d23574a7
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/52643
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

tests/gem5/insttest_se/test.py

@@ -60,7 +60,6 @@ for isa in test_progs:
                 config_args=[
                     binary,
                     cpu,
-                    "--override-download",
                     "--resource-directory",
                     resource_path,
                 ],