tools: remove blktap2 related code and documentation

Blktap2 is effectively dead for a few years.

Notable changes in this patch:

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

A patch to remove blktap2 directory will come later.

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

MAINTAINERS

@@ -192,10 +192,6 @@ F:	xen/include/asm-arm/
 F:	xen/include/public/arch-arm/
 F:	xen/include/public/arch-arm.h
 
-BLKTAP2
-S:	Orphaned
-F:	tools/blktap2/
-
 Continuous Integration (CI)
 M:	Doug Goldstein <cardoe@cardoe.com>
 W:	https://gitlab.com/xen-project/xen