[Minor][SparkR] Minor refactor and removes redundancy related to cleanClosure.

1. Only use `cleanClosure` in creation of RRDDs. Normally, user and developer do not need to call `cleanClosure` in their function definition.
2. Removes redundant code (e.g. unnecessary wrapper functions) related to `cleanClosure`.

Author: hlin09 <hlin09pu@gmail.com>

Closes #5495 from hlin09/cleanClosureFix and squashes the following commits:

74ec303 [hlin09] Minor refactor and removes redundancy.