tools/libxg: Drop stale p2m logic from ARM's meminit()

tools/libxg: Drop stale p2m logic from ARM's meminit()

Various version of gcc, when compiling with -Og, complain:

  xg_dom_arm.c: In function 'meminit':
  xg_dom_arm.c:420:19: error: 'p2m_size' may be used uninitialized in this function [-Werror=maybe-uninitialized]
    420 |     dom->p2m_size = p2m_size;
        |     ~~~~~~~~~~~~~~^~~~~~~~~~

This is actually entirely stale code since ee21f10d70^..97e34ad22d which
removed the 1:1 identity p2m for translated domains.

Drop the write of d->p2m_size, and the p2m_size local variable.  Reposition
the p2m_size field in struct xc_dom_image and correct some stale
documentation.

This change really ought to have been part of the original cleanup series.

No actual change to how ARM domains are constructed.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Julien Grall <jgrall@amazon.com>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>