Remove non-essential build flags

Removes flags that are not essential to the core of this project,
notably flags about the build mode (debug vs release, debug symbols).
These flags should be moved to platform-specific config instead.

Addresses https://github.com/ungoogled-software/ungoogled-chromium-archlinux/issues/22

flags.gn

@@ -1,10 +1,7 @@
-blink_symbol_level=0
 clang_use_chrome_plugins=false
 closure_compile=false
-enable_ac3_eac3_audio_demuxing=true
 enable_hangout_services_extension=false
 enable_hevc_demuxing=true
-enable_iterator_debugging=false
 enable_mdns=false
 enable_mse_mpeg2ts_stream_parser=true
 enable_nacl=false
@@ -17,16 +14,12 @@ enable_service_discovery=false
 enable_swiftshader=false
 enable_widevine=true
 exclude_unwind_tables=true
-fatal_linker_warnings=false
 fieldtrial_testing_like_official_build=true
 google_api_key=""
 google_default_client_id=""
 google_default_client_secret=""
-is_debug=false
-is_official_build=true
 optimize_webui=false
 safe_browsing_mode=0
-symbol_level=0
 treat_warnings_as_errors=false
 use_official_google_api_keys=false
 use_unofficial_version_number=false