Remove gl::IsInternalTextureTarget.

This function was a duplicate of ValidTexture2DDestinationTarget.
Also clean up a bit of the code in Framebuffer::completeness.

BUG=angle:660

Change-Id: Iee8d011274bfb2e426346e3ccdde7342fbb7ef44
Reviewed-on: https://chromium-review.googlesource.com/202595
Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
diff --git a/src/libGLESv2/libGLESv2.cpp b/src/libGLESv2/libGLESv2.cpp
index 6be355a..a87ff60 100644
--- a/src/libGLESv2/libGLESv2.cpp
+++ b/src/libGLESv2/libGLESv2.cpp
@@ -2709,7 +2709,7 @@
             {
                 attachmentObjectType = attachmentType;
             }
-            else if (gl::IsInternalTextureTarget(attachmentType, context->getClientVersion()))
+            else if (gl::ValidTexture2DDestinationTarget(context, attachmentType))
             {
                 attachmentObjectType = GL_TEXTURE;
             }