qa/rgw/upgrade/pacific: remove centos_8.stream.yaml and rely on ubuntu_20.04.yaml

we can't test this pacific->reef upgrade path on centos because pacific doesn't
have centos 9 builds, and reef no longer has centos 8 builds. only test
this upgrade on ubuntu focal which is still supported for both releases

this commit targets the reef branch directly because this rgw/upgrade/pacific
suite no longer exists on main and squid branches

Signed-off-by: Casey Bodley <cbodley@redhat.com>

diff --git a/qa/suites/rgw/upgrade/1-install/pacific/distro$/centos_8.stream.yaml b/qa/suites/rgw/upgrade/1-install/pacific/distro$/centos_8.stream.yaml
deleted file mode 120000 (symlink)
index e787191..0000000
--- a/qa/suites/rgw/upgrade/1-install/pacific/distro$/centos_8.stream.yaml
+++ /dev/null
@@ -1 +0,0 @@
-.qa/distros/all/centos_8.stream.yaml
\ No newline at end of file