misc varnish: do not handle bz-attachment URLs

as explained by andre_ on the comment of PS3
on https://gerrit.wikimedia.org/r/#/c/172471/3

we do not want to redirect the bugzilla-attachment
URLs to phabricator or anywhere else

so this is just cleaning up the varnish config
for disambiguity, it didn't technically matter
as long as DNS was not switched over to misc-web

Change-Id: I43c194384a539a8efc0838be0261ab7d580c7d0b
Merge "misc varnish: do not handle bz-attachment URLs" into production