Remove pandoc from bindep-fallback.txt
Remove pandoc from bindep-fallback.txt

pandoc was used in the past for building identity-api and image-api,
these are now retired.

A search using codesearch.openstack.org shows only a usage in terracotta
and that one is not invoked anywhere in infra scripts.

Corresponding change for system-config:
I83050f32dd45330831e01c9828ff75a924469f82

Change-Id: Id2e2278e160fa22e75ffbc15a8f9e233c9eeb7e9

dongwenjuan/project-config,noorul/os-project-config,openstack-infra/project-config,dongwenjuan/project-config,Tesora/tesora-project-config,openstack-infra/project-config,noorul/os-project-config,Tesora/tesora-project-config