Removed defintions of fuchsia-specific vulkan types from
vulkan_fuchsia_ext.h . Instead, the types provided by vulkan_fuchsia.h
are used now.
Also re-enabled vulkan validation on Fuchsia.

Bug: chromium:1215736
Change-Id: I795347b7fb61d90033749533cefd42cf180730fb
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2934718Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>