Remove enable_swiftshader=false from GN flags

This addresses
https://github.com/ungoogled-software/ungoogled-chromium-macos/issues/24,
which requires enable_swiftshader=true. There is no strong reason in terms
of the project's objectives to disable SwiftShader, so it will be moved
to the platform-specific flags.

flags.gn

@@ -11,7 +11,6 @@ enable_reading_list=false
 enable_remoting=false
 enable_reporting=false
 enable_service_discovery=false
-enable_swiftshader=false
 enable_widevine=true
 exclude_unwind_tables=true
 fieldtrial_testing_like_official_build=true