Remove global sources assignment filter value

With https://crrev.com/c/2418443, the files using the sources
assignment filter setting explicitly set the value. The global
initialisation from build/config/BUILDCONFIG.gn is no longer
needed.

Bug: 1018739
Change-Id: Ic5517b196213b7b3f14c902f57c8716c8e1beea3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2426564
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810642}

build/config/BUILDCONFIG.gn

@@ -296,19 +296,6 @@ is_win = current_os == "win" || current_os == "winuwp"
 is_apple = is_ios || is_mac
 is_posix = !is_win && !is_fuchsia
 
-# =============================================================================
-# SOURCES FILTERS
-# =============================================================================
-#
-# Compatibility to allow incremental migration of BUILD.gn files away from
-# depending on global call of set_sources_assignment_filter. Will be removed
-# when all files that depend on the assignment have been converted to import
-# the file directly. See https://crbug.com/1018739#c69.
-
-import("//build/config/deprecated_default_sources_assignment_filter.gni")
-sources_assignment_filter = deprecated_default_sources_assignment_filter
-set_sources_assignment_filter(sources_assignment_filter)
-
 # =============================================================================
 # TARGET DEFAULTS
 # =============================================================================