Get rid of NoValue type

[p1c2u]

NoValue type no longer needed

[codecov]

Codecov Report

Merging #340 (3c97783) into master (38ab5a0) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #340      +/-   ##
==========================================
- Coverage   95.81%   95.79%   -0.02%     
==========================================
  Files          79       78       -1     
  Lines        1623     1618       -5     
==========================================
- Hits         1555     1550       -5     
  Misses         68       68              

Impacted Files	Coverage Δ
openapi_core/casting/schemas/casters.py	96.29% <100.00%> (-0.14%)	⬇️
...penapi_core/unmarshalling/schemas/unmarshallers.py	94.73% <100.00%> (-0.10%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 38ab5a0...3c97783. Read the comment docs.