Delete extra initialization in spirv shader. Only webgl requires initialization of local variables,others don't. Extra initialization in spirv shader may affect performance. Bug: angleproject:4952 Change-Id: I004761779067748d43c1ea2630794491f1389492 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2359608 Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Tobin Ehlis <tobine@google.com> Reviewed-by: Jamie Madill <jmadill@chromium.org>