Lift restriction on calldata variables shadowing storage variables

[iamdefinitelyahuman]

What I did

Lift a restriction preventing calldata variable names from shadowing storage variable names.

Related to #2166
Fixes #2219

How I did it

Remove the checks in context and parser subpackages.

How to verify it

Run tests.

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[codecov-io]

Codecov Report

Merging #2226 (6af6fa2) into master (3667bc5) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2226      +/-   ##
==========================================
+ Coverage   85.32%   85.34%   +0.01%     
==========================================
  Files          84       84              
  Lines        8533     8528       -5     
  Branches     2064     2061       -3     
==========================================
- Hits         7281     7278       -3     
+ Misses        745      744       -1     
+ Partials      507      506       -1     

Impacted Files	Coverage Δ
vyper/context/types/function.py	83.75% <ø> (-0.21%)	⬇️
...er/function_definitions/parse_external_function.py	97.82% <ø> (+2.03%)	⬆️

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