Minor cleanups in DenseAxisArray

[blegat]

The comments says it's defined on Julia v0.7 and I can verify that it's defined in Julia v1.6 at least which is the minimal Julia version we care about.

julia> VERSION
v"1.6.7"

julia> Base.print_array
print_array (generic function with 4 methods)

[codecov]

Codecov Report

Patch coverage: 83.33% and no project coverage change.

Comparison is base (1bb7b8f) 97.97% compared to head (2698bc1) 97.97%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3429   +/-   ##
=======================================
  Coverage   97.97%   97.97%           
=======================================
  Files          34       34           
  Lines        4979     4979           
=======================================
  Hits         4878     4878           
  Misses        101      101           

Impacted Files	Coverage Δ
src/Containers/DenseAxisArray.jl	96.48% <83.33%> (ø)

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