remove obsolete Makefile

[MichaelChirico]

Just noticed this file... seems it's been here & unused a long time (it won't do anything if R is < 3.0.0? which is 7+ years ago?)

[codecov]

Codecov Report

Merging #4557 into master will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4557   +/-   ##
=======================================
  Coverage   99.61%   99.61%           
=======================================
  Files          73       73           
  Lines       14103    14109    +6     
=======================================
+ Hits        14048    14054    +6     
  Misses         55       55           

Impacted Files	Coverage Δ
src/forder.c	100.00% <0.00%> (ø)
src/openmp-utils.c	100.00% <0.00%> (ø)
R/test.data.table.R	100.00% <0.00%> (ø)

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