plancement/region_rule_cache: redundant return values for CheckAndGetCache

## Enhancement Task
For the function `CheckAndGetCache` , it returns an bool and the value in cache if check success, it will always returns false with value nil and returns true with value not nil, so could we remove the boolean value?
[https://github.com/tikv/pd/blob/c8775b6176a03eb25cb1622fc46ba1d496872df7/server/schedule/placement/region_rule_cache.go#L56-L72](https://github.com/tikv/pd/blob/c8775b6176a03eb25cb1622fc46ba1d496872df7/server/schedule/placement/region_rule_cache.go#L56-L72)