Don't assert trying to make an MSAA surface with Metal, just fail

Change-Id: I66b18d753f0e78cc1d647328322d12c037fa9ed5
Reviewed-on: https://skia-review.googlesource.com/149484
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>

src/gpu/mtl/GrMtlTextureRenderTarget.mm

@@ -39,7 +39,6 @@ GrMtlTextureRenderTarget::Make(GrMtlGpu* gpu,
                                bool isWrapped) {
     SkASSERT(nil != renderTexture);
     if (desc.fSampleCnt > 1) {
-        SkASSERT(false); // Currently don't support MSAA
         return nullptr;
     }
     SkASSERT((MTLTextureUsageShaderRead | MTLTextureUsageRenderTarget) & renderTexture.usage);