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

@@ -917,9 +917,7 @@ 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):
+    if os.path.islink(path) and not os.path.exists(path):
         os.unlink(path)