Push `validate` kwarg up to the top-level functions in `pyhf.simplemodels`

[phinate]

When doing experiments for differentiable models (#882), it's sometimes been easier to just skip validation with simplemodels, which I've been writing out specs for. This just reduces a bit of code overhead for me when I do that ;)

Made sure to highlight in the docstring that this should probably never be touched if one doesn't know why.

This is also partially because #1665 isn't fully done yet, which would probably make this redundant in some ways.

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[kratsg]

Since master allows for custom specs - why not use a custom "catch-all" spec like type: object and no other details?

[codecov[bot]]

Codecov Report

Base: 98.24% // Head: 98.24% // No change to project coverage :thumbsup:

Coverage data is based on head (98d788a) compared to base (c68e43b). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1858   +/-   ##
=======================================
  Coverage   98.24%   98.24%           
=======================================
  Files          68       68           
  Lines        4378     4378           
  Branches      726      726           
=======================================
  Hits         4301     4301           
  Misses         45       45           
  Partials       32       32           

Flag	Coverage Δ
contrib	26.58% <50.00%> (ø)
doctest	60.57% <100.00%> (ø)
unittests-3.10	96.13% <100.00%> (ø)
unittests-3.7	96.12% <100.00%> (ø)
unittests-3.8	96.16% <100.00%> (ø)
unittests-3.9	96.18% <100.00%> (ø)

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Impacted Files	Coverage Δ
src/pyhf/simplemodels.py	92.30% <100.00%> (ø)

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