!8350 delete useless import in rmsprop optimizer

From: @wangnan39
Reviewed-by: @zh_qh,@HilbertDavid
Signed-off-by: @HilbertDavid

mindspore/nn/optim/rmsprop.py

@@ -15,7 +15,6 @@
 """rmsprop"""
 from mindspore.ops import functional as F, composite as C, operations as P
 from mindspore._checkparam import Validator as validator
-from mindspore._checkparam import Rel
 from .optimizer import Optimizer
 
 _rmsprop_opt = C.MultitypeFuncGraph("rmsprop_opt")