arm64: mm: remove redundant definitions physvirt_offset & vmemmap

The commit 0edc78af

(arm64: mm: use single quantity to represent the PA to VA translation)
has removed the definitions physvirt_offset & vmemmap, however which are still
on there since the wrong code merging and caused the compiling error.

Signed-off-by: Xulin Sun <xulin.sun@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>