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>

.gitignore

@@ -98,19 +98,6 @@ tools/libs/evtchn/headers.chk
 tools/libs/gnttab/headers.chk
 tools/libs/call/headers.chk
 tools/libs/foreignmemory/headers.chk
-tools/blktap2/daemon/blktapctrl
-tools/blktap2/drivers/img2qcow
-tools/blktap2/drivers/lock-util
-tools/blktap2/drivers/qcow-create
-tools/blktap2/drivers/qcow2raw
-tools/blktap2/drivers/tapdisk
-tools/blktap2/drivers/tapdisk-client
-tools/blktap2/drivers/tapdisk-diff
-tools/blktap2/drivers/tapdisk-stream
-tools/blktap2/drivers/tapdisk2
-tools/blktap2/drivers/td-util
-tools/blktap2/vhd/vhd-update
-tools/blktap2/vhd/vhd-util
 tools/console/xenconsole
 tools/console/xenconsoled
 tools/console/client/_paths.h
@@ -326,7 +313,6 @@ tools/libxl/*.pyc
 tools/libxl/libxl-save-helper
 tools/libxl/test_timedereg
 tools/libxl/test_fdderegrace
-tools/blktap2/control/tap-ctl
 tools/firmware/etherboot/eb-roms.h
 tools/firmware/etherboot/gpxe-git-snapshot.tar.gz
 tools/misc/xenwatchdogd