performance: reduce system calls required for remove_dead_links

`os.path.exists()` will report False if the target of a symlink doesn't
exist, so we can avoid a costly call to realpath here.

lib/spack/llnl/util/filesystem.py

@@ -917,10 +917,8 @@ def remove_if_dead_link(path):
     Parameters:
         path (str): The potential dead link
     """
-    if os.path.islink(path):
-        real_path = os.path.realpath(path)
-        if not os.path.exists(real_path):
-            os.unlink(path)
+    if os.path.islink(path) and not os.path.exists(path):
+        os.unlink(path)
 
 
 def remove_linked_tree(path):