billing: Remove references to legacy stripe_fixtures.json.
billing: Remove references to legacy stripe_fixtures.json.

tommyip/zulip,timabbott/zulip,zulip/zulip,zulip/zulip,tommyip/zulip,timabbott/zulip,shubhamdhama/zulip,punchagan/zulip,andersk/zulip,shubhamdhama/zulip,synicalsyntax/zulip,hackerkid/zulip,jackrzhang/zulip,showell/zulip,andersk/zulip,timabbott/zulip,jackrzhang/zulip,timabbott/zulip,zulip/zulip,rishig/zulip,showell/zulip,dhcrzf/zulip,synicalsyntax/zulip,zulip/zulip,shubhamdhama/zulip,dhcrzf/zulip,hackerkid/zulip,eeshangarg/zulip,andersk/zulip,punchagan/zulip,tommyip/zulip,eeshangarg/zulip,punchagan/zulip,brainwane/zulip,rht/zulip,brainwane/zulip,punchagan/zulip,brainwane/zulip,brainwane/zulip,hackerkid/zulip,andersk/zulip,dhcrzf/zulip,jackrzhang/zulip,hackerkid/zulip,shubhamdhama/zulip,timabbott/zulip,andersk/zulip,kou/zulip,rishig/zulip,eeshangarg/zulip,showell/zulip,rishig/zulip,jackrzhang/zulip,shubhamdhama/zulip,jackrzhang/zulip,punchagan/zulip,rht/zulip,rishig/zulip,andersk/zulip,eeshangarg/zulip,synicalsyntax/zulip,jackrzhang/zulip,kou/zulip,brainwane/zulip,tommyip/zulip,jackrzhang/zulip,rht/zulip,synicalsyntax/zulip,kou/zulip,showell/zulip,rishig/zulip,dhcrzf/zulip,showell/zulip,kou/zulip,andersk/zulip,kou/zulip,kou/zulip,synicalsyntax/zulip,synicalsyntax/zulip,eeshangarg/zulip,rishig/zulip,showell/zulip,zulip/zulip,tommyip/zulip,rht/zulip,dhcrzf/zulip,brainwane/zulip,punchagan/zulip,timabbott/zulip,shubhamdhama/zulip,zulip/zulip,hackerkid/zulip,eeshangarg/zulip,rishig/zulip,hackerkid/zulip,showell/zulip,brainwane/zulip,synicalsyntax/zulip,rht/zulip,kou/zulip,eeshangarg/zulip,tommyip/zulip,hackerkid/zulip,rht/zulip,dhcrzf/zulip,tommyip/zulip,timabbott/zulip,rht/zulip,punchagan/zulip,zulip/zulip,shubhamdhama/zulip,dhcrzf/zulip