Remove all the ImageSharp.Drawing project and related tests.

[tocsoft]

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Description

ImageSharp.Drawing has been removed form the main repository and moved over to https://github.com/SixLabors/ImageSharp.Drawing (which also has had Shapes merged into it too).

This PR is just removing all drawing extensions minus the DrawImage() one (as I feel its important enough to be required without vector drawing etc.) that make ImageSharp.Drawing just the vector/text/brush based items.

[antonfirsov]

What's the fate of Drawing benchmarks? (ImageSharp.Benchmarks/Drawing/** - can't find them in the new repo)
Don't we consider them valuable any longer, or are they left out by mistake?

[JimBobSquarePants]

@antonfirsov I'll open a separate PR in Drawing to add the benchmarks

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@1924acb). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master    #1082   +/-   ##
=========================================
  Coverage          ?   87.33%           
=========================================
  Files             ?     1131           
  Lines             ?    50041           
  Branches          ?     3677           
=========================================
  Hits              ?    43703           
  Misses            ?     5550           
  Partials          ?      788

Impacted Files	Coverage Δ
....Tests/TestUtilities/Tests/TestEnvironmentTests.cs	70.58% <ø> (ø)
...rocessing/Processors/Drawing/DrawImageProcessor.cs	100% <ø> (ø)
...awing/Processing/Extensions/DrawImageExtensions.cs	95.65% <ø> (ø)
tests/ImageSharp.Tests/GraphicsOptionsTests.cs	100% <ø> (ø)
...ts/ImageSharp.Tests/Image/ImageTests.WrapMemory.cs	83.33% <100%> (ø)
tests/ImageSharp.Tests/Drawing/DrawImageTests.cs	98.97% <100%> (ø)
...ests/TestUtilities/ImageProviders/SolidProvider.cs	58.62% <100%> (ø)

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