attributes - links - Get rid of attributes with links to Che/CRW docs

Upstream (Che) & downstream (CRW) docs attributes contain a number of links to the same docs set. These were pbbly meant to ease downstreaming but are in fact confusing and non-systemic. The translation of upstream links to downstream links is done by the downstreaming process.

Example:

# upstream:
link-installing-an-instance: link:{site-baseurl}che-7/che-quick-starts/[Che 'quick-starts']

# downstream:
:link-installing-an-instance: link:{prod-ig-url}installing_codeready_workspaces_on_openshift_container_platform[Installing CodeReady Workspaces on OpenShift Container Platform]

1. Remove these attributes from definition and content files.
2. Verify that downstreaming handles their conversion well.