shared/install: in install_context_mark_for_removal ignore not found units

shared/install: in install_context_mark_for_removal ignore not found units

With the following test case:
[Install]
WantedBy= default.target
Also=foobar-unknown.service

disabling would fail with:
$ ./systemctl --root=/ disable testing.service
Cannot find unit foobar-unknown.service.       # this is level debug
Failed to disable: No such file or directory.  # this is the error

After the change we proceed:
$ ./systemctl --root=/ disable testing.service
Cannot find unit foobar-unknown.service.
Removed /etc/systemd/system/default.target.wants/testing.service.

This does not affect specifying a missing unit directly:
$ ./systemctl --root=/ disable nosuch.service
Failed to disable: No such file or directory.