gdb/python: remove unneeded nullptr check in frapy_block

Spotted a redundant nullptr check in python/py-frame.c in the function
frapy_block.  This was introduced in commit 57126e4a45 when we
expanded an earlier check in return early if the pointer in question
is nullptr.

There should be no user visible changes after this commit.

Reviewed-By: Tom Tromey <tom@tromey.com>

gdb/python/py-frame.c

@@ -304,13 +304,7 @@ frapy_block (PyObject *self, PyObject *args)
       return NULL;
     }
 
-  if (block)
-    {
-      return block_to_block_object
-	(block, fn_block->function ()->objfile ());
-    }
-
-  Py_RETURN_NONE;
+  return block_to_block_object (block, fn_block->function ()->objfile ());
 }