chroot: keep ownership of cache_git (!1845)

Remove /home/pmos/git from pmb.config.chroot_home_symlinks, so
pmb.chroot.init.init() will not change the ownership of the
$WORK/cache_git dir.

The symlink was added two years ago in [1] as compatibility to previous
behavior, because the mountpoint of cache_git was changed from
/home/user/git to /mnt/pmbootstrap-git. Forcing the ownership of that
dir to be the pmos user inside the chroot made sense, because the only
cloned repository (aports_upstream) would also be owned by the pmos
user. With the upcoming change to have all cloned repos owned by the
host-system's user, this is just confusing.

The /home/pmos/git symlink does not seem to be used by anybody, it is
not mentioned in the pmbootstrap code and in the wiki.

[1] b4dd7a89 "Close #709: Improve user creation (#725)"
    https://github.com/postmarketOS/pmbootstrap/pull/725

pmb/config/__init__.py

@@ -137,7 +137,6 @@ chroot_mount_bind = {
 chroot_home_symlinks = {
     "/mnt/pmbootstrap-abuild-config": "/home/pmos/.abuild",
     "/mnt/pmbootstrap-ccache": "/home/pmos/.ccache",
-    "/mnt/pmbootstrap-git": "/home/pmos/git",
     "/mnt/pmbootstrap-packages": "/home/pmos/packages/pmos",
 }