tests/migration: Drop arch_[source|target]

tests/migration: Drop arch_[source|target]

Coverity complained about them. These two variables are never used now
after commit 832c732c5d ("migration-test: Create arch_opts"), and/or commit
34cc54fb35 ("tests/qtest/migration-test: Use custom asm bios for ppc64").

Resolves: Coverity CID 1568379
Resolves: Coverity CID 1568380
Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20241216161413.1644171-4-peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>