Remove code that was for Julia v0.6

[blegat]

No description provided.

[coveralls]

Coverage decreased (-23.2%) to 76.446% when pulling 3100808 on bl/subsv0.6 into 2a03f8e on master.

[codecov-io]

Codecov Report

Merging #39 into master will decrease coverage by 23.19%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #39      +/-   ##
==========================================
- Coverage   99.63%   76.44%   -23.2%     
==========================================
  Files          14       14              
  Lines         553      726     +173     
==========================================
+ Hits          551      555       +4     
- Misses          2      171     +169

Impacted Files	Coverage Δ
src/subs.jl	87.93% <100%> (-12.07%)	⬇️
src/div.jl	64.7% <0%> (-35.3%)	⬇️
src/operators.jl	65.11% <0%> (-34.89%)	⬇️
src/term.jl	66.66% <0%> (-33.34%)	⬇️
src/poly.jl	65.54% <0%> (-33.19%)	⬇️
src/mult.jl	70.49% <0%> (-29.51%)	⬇️
src/diff.jl	70.58% <0%> (-29.42%)	⬇️
src/var.jl	74% <0%> (-26%)	⬇️
src/monovec.jl	74.61% <0%> (-24.37%)	⬇️
src/ncmult.jl	79.36% <0%> (-20.64%)	⬇️
... and 4 more

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