Clean up deprecated sources assignment filter in //build

Remove deprecated_default_sources_assignment_filter and replace
it with explicit is_win, is_mac, etc. sections.

Bug: 1018739
Change-Id: If53a8ca4e8ca50ecd84f0b4b506191ad008a3b4b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2443094
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814204}

build/win/BUILD.gn

@@ -6,13 +6,6 @@ import("//build/config/clang/clang.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 import("//build/config/win/manifest.gni")
 
-# This file depends on the legacy global sources assignment filter. It should
-# be converted to check target platform before assigning source files to the
-# sources variable. Remove this import and set_sources_assignment_filter call
-# when the file has been converted. See https://crbug.com/1018739 for details.
-import("//build/config/deprecated_default_sources_assignment_filter.gni")
-set_sources_assignment_filter(deprecated_default_sources_assignment_filter)
-
 # Depending on this target will cause the manifests for Chrome's default
 # Windows and common control compatibility and elevation for executables.
 windows_manifest("default_exe_manifest") {