drop requirements.in

drop requirements.in

dependabot adds an entry in generated requirements.txt like
```
-e file:///home/dependabot/dependabot-updater/tmp/dependabot_20200617-72-1n8af4b  # via -r requirements.in
```

this is expected. as dependabot does not read docs/INSTALL.rst, hence
failed to apply
```
sed '/^-e / d' to the generated file.

instead of teaching dependabot to read the manual, it's simpler
just ditch requirements.in .

see also #1367

* update-requirements.sh: add a helper script for updating
  requirements.txt
* requirements.in: removed
* docs/INSTALL.rst: updated to point user to the new
  helper script

Signed-off-by: Kefu Chai <kchai@redhat.com>