docs: remove Debian 7 support
docs: remove Debian 7 support

I've added a separate entry to faq.md in the website that lists unsupported
distributions.

Change-Id: I12473877596b096939704b5913b5302190a81789
Reviewed-on: http://gerrit.cloudera.org:8080/1898
Tested-by: Kudu Jenkins
Reviewed-by: Jean-Daniel Cryans

helifu/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,andrwng/kudu,andrwng/kudu,helifu/kudu,cloudera/kudu,InspurUSA/kudu,helifu/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,helifu/kudu,InspurUSA/kudu,helifu/kudu,andrwng/kudu,helifu/kudu,EvilMcJerkface/kudu,andrwng/kudu,cloudera/kudu,cloudera/kudu,cloudera/kudu,andrwng/kudu,helifu/kudu,InspurUSA/kudu,cloudera/kudu,InspurUSA/kudu,cloudera/kudu,andrwng/kudu,cloudera/kudu,EvilMcJerkface/kudu,cloudera/kudu,andrwng/kudu,InspurUSA/kudu,InspurUSA/kudu,InspurUSA/kudu,helifu/kudu,andrwng/kudu,InspurUSA/kudu,cloudera/kudu,helifu/kudu,andrwng/kudu,EvilMcJerkface/kudu