Remove swiftshader_spirv_tools_dir

SwiftShader now uses an independent build of SPIRV-Tools dependencies,
like in other environments.

Bug: b/158002593
Change-Id: I06df3d9be0f5aa8b253c0544a51dbc78d293d9b4
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3569623
Auto-Submit: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>

build_overrides/swiftshader.gni

@@ -7,6 +7,3 @@ swiftshader_standalone = false
 
 # Path to SwiftShader
 swiftshader_dir = "//third_party/SwiftShader"
-
-# Paths to SwiftShader dependencies
-swiftshader_spirv_tools_dir = "//third_party/vulkan-deps/spirv-tools/src"