tools: remove blktap2 related code and documentation

tools: remove blktap2 related code and documentation

Blktap2 is effectively dead code for a few years.

Notable changes in this patch:

0. Unhook blktap2 from build system
1. Now libxl no longer supports TAP disk backend, appropriate assertions
   are added and some code paths now return ERROR_FAIL
2. Tap is no longer a supported backend in doc
3. Remove relevant entries in MAINTAINERS

A patch to actually remove blktap2 directory will come later.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: George Dunlap <George.Dunlap@eu.citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>