xml: remove impl Context

the constructors do not buy us anything
7 jobs for xml in 26 minutes and 29 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Test
passed #174893
fedora test

00:06:32

 
  Lint
manual #174895
allowed to fail manual
clippy
passed #174894
rustfmt

00:00:31

passed #174896
tarball

00:10:10

 
  Cross Distro
passed #174899
debian_i386:testing

00:07:10

passed #174898
debian:testing

00:06:37

passed #174897
fedora:rawhide

00:09:46