Also remove unused `lane` parameter from `Shadow::get`

Fixes: b/173788646
Change-Id: I6797ccd471da4c5b639b2ae52d1a6d2ca9a56522
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/50629Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Jaebaek Seo <jaebaek@google.com>