layers: Remove default ctors for {Image,}SamplerDescriptor

These just added confusion; we already support the !immut case in the other
ctor.

V2: ImageSamplerDescriptor remaining ctor was buggy.
diff --git a/layers/descriptor_sets.cpp b/layers/descriptor_sets.cpp
index b4dcfcf..3539a83 100644
--- a/layers/descriptor_sets.cpp
+++ b/layers/descriptor_sets.cpp
@@ -331,7 +331,7 @@
                         descriptors_.emplace_back(new SamplerDescriptor(immut_sampler + di));
                         some_update_ = true;  // Immutable samplers are updated at creation
                     } else
-                        descriptors_.emplace_back(new SamplerDescriptor());
+                        descriptors_.emplace_back(new SamplerDescriptor(nullptr));
                 }
                 break;
             }
@@ -342,7 +342,7 @@
                         descriptors_.emplace_back(new ImageSamplerDescriptor(immut + di));
                         some_update_ = true;  // Immutable samplers are updated at creation
                     } else
-                        descriptors_.emplace_back(new ImageSamplerDescriptor());
+                        descriptors_.emplace_back(new ImageSamplerDescriptor(nullptr));
                 }
                 break;
             }
@@ -742,11 +742,6 @@
     }
 }
 
-cvdescriptorset::SamplerDescriptor::SamplerDescriptor() : sampler_(VK_NULL_HANDLE), immutable_(false) {
-    updated = false;
-    descriptor_class = PlainSampler;
-};
-
 cvdescriptorset::SamplerDescriptor::SamplerDescriptor(const VkSampler *immut) : sampler_(VK_NULL_HANDLE), immutable_(false) {
     updated = false;
     descriptor_class = PlainSampler;
@@ -951,14 +946,8 @@
     }
 }
 
-cvdescriptorset::ImageSamplerDescriptor::ImageSamplerDescriptor()
-    : sampler_(VK_NULL_HANDLE), immutable_(false), image_view_(VK_NULL_HANDLE), image_layout_(VK_IMAGE_LAYOUT_UNDEFINED) {
-    updated = false;
-    descriptor_class = ImageSampler;
-}
-
 cvdescriptorset::ImageSamplerDescriptor::ImageSamplerDescriptor(const VkSampler *immut)
-    : sampler_(VK_NULL_HANDLE), immutable_(true), image_view_(VK_NULL_HANDLE), image_layout_(VK_IMAGE_LAYOUT_UNDEFINED) {
+    : sampler_(VK_NULL_HANDLE), immutable_(false), image_view_(VK_NULL_HANDLE), image_layout_(VK_IMAGE_LAYOUT_UNDEFINED) {
     updated = false;
     descriptor_class = ImageSampler;
     if (immut) {