fix variable with melt(measure.vars=list), na.rm=T/F consistency

[tdhock]

Closes #4455
hi @jangorecki tagging you for a review since this is a fix for #4455 which you labeled.
Turns out the fix was really easy, just had to delete two lines of code.
I had to update the expected values for two existing tests, which I suspect were created based on the empirical output of melt, rather than the theoretically expected values.
In the linked issue the example code which illustrates the problem is:

library(data.table)
wide.dt <- data.table(
  family=1,
  age_child1=NA_real_, sex_child1=NA_character_,
  age_child2=5, sex_child2="m")
measure.vars <- list(age=c(2,4), sex=c(3,5))
(na.rm.T <- melt(wide.dt, measure.vars=measure.vars, na.rm=TRUE))
(na.rm.F <- melt(wide.dt, measure.vars=measure.vars, na.rm=FALSE)[!is.na(age)])

on current master we get results that are inconsistent (na.rm=TRUE gives variable=1, but na.rm=FALSE gives variable=2).

> (na.rm.T <- melt(wide.dt, measure.vars=measure.vars, na.rm=TRUE))

   family variable age sex
1:      1        1   5   m
> (na.rm.F <- melt(wide.dt, measure.vars=measure.vars, na.rm=FALSE)[!is.na(age)])
   family variable age sex
1:      1        2   5   m

This is problematic for downstream applications such as tdhock/nc#11 which need variable to be the corresponding index of the measure.vars list elements. In the example above we need variable=2 in both cases so that we know the data come from child2.

After applying this PR I get

> (na.rm.T <- melt(wide.dt, measure.vars=measure.vars, na.rm=TRUE))
   family variable age sex
1:      1        2   5   m
> (na.rm.F <- melt(wide.dt, measure.vars=measure.vars, na.rm=FALSE)[!is.na(age)])
   family variable age sex
1:      1        2   5   m

All tests pass on my computer (R-4.0.2, Ubuntu 18.04), please code review.

[codecov]

Codecov Report

Merging #4723 (54afdc2) into master (7b4bd7d) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4723      +/-   ##
==========================================
+ Coverage   99.42%   99.43%   +0.01%     
==========================================
  Files          73       73              
  Lines       14466    14460       -6     
==========================================
- Hits        14383    14379       -4     
+ Misses         83       81       -2     

Impacted Files	Coverage Δ
src/fmelt.c	99.42% <100.00%> (+0.37%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7b4bd7d...54afdc2. Read the comment docs.

[tdhock]

hi again @mattdowle this is a pretty simple fix, any chance you could review and consider including in next release?

[tdhock]

hi again @mattdowle @arunsrinivasan please consider merging this simple fix for melt for the next release, thanks! (sorry if I'm being annoying, I know there are a lot of other PRs to examine, I just wanted to make clear that my suggestion for prioritizing my melt-related PRs should be first this one, then #4720, then #4731, then #4737)