Formally remove relationship to reshape2

[MichaelChirico]

Closes #3549
A first pass at removing reshape2 where possible.

The else branches in melt and dcast may or may not still be relevant... I checked the referenced package (edarf) and there's no mention of reshape2 in its current DESCRIPTION; anyway I imagine revdep checks for other packages will come back problematic.

Still we should at least start the deprecation process of support in the next release... doesn't make much sense to me to maintain this code complexity for a deprecated package.

[jangorecki]

Instead of removing tests I would move them into integration tests, which are not being run on CRAN, but we do run them in CI.
https://github.com/Rdatatable/data.table/blob/master/inst/tests/tests-DESCRIPTION
https://github.com/Rdatatable/data.table/blob/master/inst/tests/other.Rraw

[MichaelChirico]

@jangorecki no tests removed here. One test was edited since the error message had reshape2 in it (this was changed in fmelt.c)

[codecov]

Codecov Report

Merging #3577 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3577      +/-   ##
==========================================
- Coverage   97.58%   97.58%   -0.01%     
==========================================
  Files          66       66              
  Lines       12695    12693       -2     
==========================================
- Hits        12389    12387       -2     
  Misses        306      306

Impacted Files	Coverage Δ
R/fcast.R	99.4% <100%> (-0.01%)	⬇️
src/fmelt.c	88.63% <100%> (ø)	⬆️
R/fmelt.R	100% <100%> (ø)	⬆️

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[codecov]

Codecov Report

Merging #3577 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3577      +/-   ##
==========================================
- Coverage   97.58%   97.58%   -0.01%     
==========================================
  Files          66       66              
  Lines       12695    12693       -2     
==========================================
- Hits        12389    12387       -2     
  Misses        306      306

Impacted Files	Coverage Δ
R/fcast.R	99.4% <100%> (-0.01%)	⬇️
src/fmelt.c	88.63% <100%> (ø)	⬆️
R/fmelt.R	100% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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