Don't create custom literal parser by default

[rafalp]

I've did deep dive into GraphQL's query executor and found that if scalar has no literal parser, GraphQL executor will use default one, which did exactly what our default one, eg. convert AST value to python type, then call parse_value for field.

In this PR I've added tests cementing this behavior in the core, and removed our fallback default literal parser. I'll also open issue on our docs to remember to update that parse_literal is not required if your scalar doesn't need custom AST unpacking logic.

Fixes #371

[codecov]

Codecov Report

Merging #387 into master will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #387      +/-   ##
==========================================
- Coverage   98.16%   98.16%   -0.01%     
==========================================
  Files         105      105              
  Lines        5078     5063      -15     
==========================================
- Hits         4985     4970      -15     
  Misses         93       93              

Impacted Files	Coverage Δ
ariadne/scalars.py	100.00% <ø> (ø)
tests/django/test_scalars.py	100.00% <ø> (ø)
tests/test_custom_scalars.py	100.00% <100.00%> (ø)

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[bogdal]

LGTM 👍