Remove feedback loop support from back-end.

Front-end detection still in place and will be removed in a follow-up.
Removes the Vulkan feature and the special clear handling.

Bug: angleproject:4959
Change-Id: I5d44c3f7dbdb49d8aa0375f54b7148df09732ba2
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2363208
Reviewed-by: Tobin Ehlis <tobine@google.com>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
diff --git a/src/tests/gl_tests/FramebufferTest.cpp b/src/tests/gl_tests/FramebufferTest.cpp
index 699b3ea..dbd20ef 100644
--- a/src/tests/gl_tests/FramebufferTest.cpp
+++ b/src/tests/gl_tests/FramebufferTest.cpp
@@ -1658,11 +1658,9 @@
 // The test emulates the read-only feedback loop in Manhattan.
 TEST_P(FramebufferTest_ES3, ReadOnlyDepthFeedbackLoopSupported)
 {
-    // Feedback loops not supported on D3D11 and may not ever be.
-    ANGLE_SKIP_TEST_IF(IsD3D11());
-
-    // Also this particular test doesn't work on Android despite similar support in Manhattan.
-    ANGLE_SKIP_TEST_IF(IsAndroid() && IsOpenGLES());
+    // Feedback loops are only supported on Vulkan.
+    // TODO(jmadill): Make GL extension. http://anglebug.com/4969
+    ANGLE_SKIP_TEST_IF(!IsVulkan());
 
     constexpr GLuint kSize = 2;
     glViewport(0, 0, kSize, kSize);
@@ -1725,11 +1723,9 @@
 // Tests corner cases with read-only depth-stencil feedback loops.
 TEST_P(FramebufferTest_ES3, ReadOnlyDepthFeedbackLoopStateChanges)
 {
-    // Feedback loops not supported on D3D11 and may not ever be.
-    ANGLE_SKIP_TEST_IF(IsD3D11());
-
-    // Also this particular test doesn't work on Android despite similar support in Manhattan.
-    ANGLE_SKIP_TEST_IF(IsAndroid() && IsOpenGLES());
+    // Feedback loops are only supported on Vulkan.
+    // TODO(jmadill): Make GL extension. http://anglebug.com/4969
+    ANGLE_SKIP_TEST_IF(!IsVulkan());
 
     constexpr GLuint kSize = 2;
     glViewport(0, 0, kSize, kSize);