Update ImageSharp.Drawing.sln to Visual Studio 18

Removed Visual Studio Version 17 entry from solution file.

Author: JimBobSquarePants

ImageSharp.Drawing.sln

@@ -2,8 +2,6 @@
 Microsoft Visual Studio Solution File, Format Version 12.00
 # Visual Studio Version 18
 VisualStudioVersion = 18.0.11123.170
-# Visual Studio Version 17
-VisualStudioVersion = 17.14.36623.8 d17.14
 MinimumVisualStudioVersion = 10.0.40219.1
 Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "_root", "_root", "{C317F1B1-D75E-4C6D-83EB-80367343E0D7}"
 	ProjectSection(SolutionItems) = preProject