When removing a file from a view, don't fail if it doesn't exist (#12960)

Sometimes when remove_file is called on a link, that link is missing
(perhaps ctrl-C happened halfway through a previous action). As
removing a non-existent file is no problem, this patch changes the
behavior so Spack continues rather than stopping with an error.

Currently you would see

  ValueError: /path/to/dir is not a link tree!

and now it continues with a warning.

lib/spack/spack/filesystem_view.py

@@ -357,6 +357,9 @@ def unmerge(self, spec, ignore=None):
         tree.unmerge_directories(view_dst, ignore_file)
 
     def remove_file(self, src, dest):
+        if not os.path.lexists(dest):
+            tty.warn("Tried to remove %s which does not exist" % dest)
+            return
         if not os.path.islink(dest):
             raise ValueError("%s is not a link tree!" % dest)
         # remove if dest is a hardlink/symlink to src; this will only