Remove analyzer severity setting

[JimBobSquarePants]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
• I have verified that I am following matches the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules 👮.
• I have provided test coverage for my change (where applicable)

Description

Will fix SixLabors/ImageSharp#1658

See
RehanSaeed/EditorConfig#56

And
dotnet/roslyn#43051 (comment)

For more info

[codecov]

Codecov Report

Merging #20 (5fe9a49) into master (1f7ee70) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #20   +/-   ##
=======================================
  Coverage   19.77%   19.77%           
=======================================
  Files           4        4           
  Lines         359      359           
  Branches       88       88           
=======================================
  Hits           71       71           
  Misses        288      288           

Flag	Coverage Δ
unittests	19.77% <ø> (ø)

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