No longer activate on the presence of .sln or .slnf files

Solution files can be used for languages other than C#. Any solution file that will result in a usable C# workspace must reference .csproj files. We already search for .csproj files as part of the activation event.

Resolves #5794

Author: JoeRobich

package.json

@@ -957,7 +957,7 @@
     "onCommand:o.showOutput",
     "onCommand:omnisharp.registerLanguageMiddleware",
     "workspaceContains:project.json",
-    "workspaceContains:**/*.{csproj,sln,slnf,csx,cake}"
+    "workspaceContains:**/*.{csproj,csx,cake}"
   ],
   "contributes": {
     "themes": [
@@ -5679,4 +5679,4 @@
       }
     ]
   }
-}
+}