tools/libx{l, c}: Remove the toolstack_{save, restore} callbacks

tools/libx{l, c}: Remove the toolstack_{save, restore} callbacks

Update the libxc spec to indicate more sternly that TOOLSTACK records
should no longer be used.

Also, trim further toolstack infrastructure which should have gone in
c/s 39bf4e9 "tools/libxl: Drop all knowledge of toolstack callbacks"

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>