[R-package] lgb.prepare() and lgb.prepare2() should be simplified

[jameslamb]

Summary

To close this issue, propose a PR that reduces duplicated code in lgb.prepare() and lgb.prepare2().

Motivation

lgb.prepare() and lgb.prepare2() currently contain a lot of duplicated code and could be simplified. For example, some code is duplicated in the data.frame and data.table blocks of those functions, such as this snippet

list_classes <- sapply(data, class)
is_char <- which(list_classes == "character")
is_fact <- c(which(list_classes == "factor"), is_char)

[jameslamb]

Closed in favor of being in #2302. We decided to keep all feature requests in one place.

Welcome to contribute this feature! Please re-open this issue (or post a comment if you are not a topic starter) if you are actively working on implementing this feature.

[jameslamb]

This issue can be considered permanently closed! Now that #3095 has been merged, there is only one such function (lgb.convert()), so this issue is now irrelevant.