[R-package] fix inaccurate comments, remove unnecessary comments

[jameslamb]

This PR proposes a few changes to comments in the source code of the R package. Some things I've noticed while looking closely at that code a lot recently, working on #4310.

• removes reference to lgb.convert() (was removed about a year ago, in [R-package] refactor and improvements to lgb.convert() functions (fixes #2678, #2681) #3269)
  • I also checked for other uses with git grep -E "lgb\.convert"...did not find any
• removes comments that do not add any additional information about what the code is doing or why it's doing it (similar to changes in [R-package] remove unnecessary comments #4383)

for example:

# Check if dataset is not a dataset
if (!lgb.is.Dataset(x = dataset)) {

[StrikerRUS]

Thanks, nice cleanup!

I also checked for other uses with git grep -E "lgb\.convert"...did not find any

Confirmed.

[github-actions]

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