BUILD.gn: Remove XCB sources for chromeos.
BUILD.gn: Remove XCB sources for chromeos.

VK_USE_PLATFORM_XCB_KHR is no longer defined for chromeos.

Bug: b/209030951
Change-Id: Iad28747fb06e13126ea83e1a38b15293a945bcbb
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/61550
Reviewed-by: Nicolas Capens <51edc787c30f24c4904e8ffbb5523c3a9f7a451d@google.com>
Commit-Queue: Ben Clayton <1f693eb7158dc33015048c09d6712f74dabddf79@google.com>
Tested-by: Ben Clayton <1f693eb7158dc33015048c09d6712f74dabddf79@google.com>
Kokoro-Result: Ben Clayton <1f693eb7158dc33015048c09d6712f74dabddf79@google.com>

google/swiftshader,google/swiftshader,google/swiftshader