Add csc as default compilerName if one is not specified

Also remove csc from project.json files, since it is no longer
necessary.

src/Microsoft.DotNet.Cli.Utils/project.json

@@ -1,6 +1,5 @@
 {
     "version": "1.0.0-*",
-    "compilerName": "csc",
 
     "shared": "**/*.cs",
 

src/Microsoft.DotNet.Cli/project.json

@@ -1,7 +1,6 @@
 {
   "name": "dotnet",
   "version": "1.0.0-*",
-  "compilerName": "csc",
   "compilationOptions": {
     "emitEntryPoint": true
   },

src/Microsoft.DotNet.Tools.Compiler.Csc/project.json

@@ -1,7 +1,6 @@
 {
   "name": "dotnet-compile-csc",
   "version": "1.0.0-*",
-  "compilerName": "csc",
   "compilationOptions": {
     "emitEntryPoint": true
   },

src/Microsoft.DotNet.Tools.Compiler/Program.cs

@@ -191,11 +191,7 @@ namespace Microsoft.DotNet.Tools.Compiler
             }
 
             var compilerName = context.ProjectFile.CompilerName;
-            if (compilerName == null)
-            {
-                Console.Error.WriteLine("Could not find the compiler name. Please specify it in the project.json file.");
-                return false;
-            }
+            compilerName = compilerName ?? "csc";
 
             // Write RSP file
             var rsp = Path.Combine(intermediateOutputPath, $"dotnet-compile.{compilerName}.rsp");

src/Microsoft.DotNet.Tools.Compiler/project.json

@@ -1,7 +1,6 @@
 {
   "name": "dotnet-compile",
   "version": "1.0.0-*",
-  "compilerName": "csc",
   "compilationOptions": {
     "emitEntryPoint": true
   },

src/Microsoft.DotNet.Tools.Publish/project.json

@@ -1,7 +1,6 @@
 {
   "name": "dotnet-publish",
   "version": "1.0.0-*",
-  "compilerName": "csc",
   "compilationOptions": {
     "emitEntryPoint": true
   },

src/Microsoft.DotNet.Tools.Resgen/project.json

@@ -1,7 +1,6 @@
 {
   "name": "resgen",
   "version": "1.0.0-*",
-  "compilerName": "csc",
   "compilationOptions": {
     "emitEntryPoint": true
   },

src/Microsoft.Extensions.ProjectModel/project.json

@@ -1,7 +1,6 @@
 {
   "version": "1.0.0-*",
   "description": "Types to model a .NET Project",
-  "compilerName": "csc",
   "dependencies": {
     "Microsoft.CSharp": "4.0.1-beta-23419",
     "System.Collections": "4.0.11-beta-23419",