Vulkan: Don't access the XFB object during link The descriptor set layout set up for XFB emulation lived in TransfromFeedbackVk, but did not access anything in that object at all. More importantly, it doesn't make any sense to look at the current XFB object when linking a program. Bug: angleproject:8297 Change-Id: I8bd0e747b7d3e3783ef053f20378cc42d2c1d997 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4774786 Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Charlie Lao <cclao@google.com> Reviewed-by: Yuxin Hu <yuxinhu@google.com>