Remove 'syntax = proto2' from .proto files.

That marker is only recognized by pb3-compatible compilers, meaning that
tools which are otherwise compatible with our files break on them.
Notably, this includes hprotoc (for haskell).

common/pb/admin_commands.proto

-syntax = "proto2";
 message AdminCommand {
     enum AdminCommandType {
         UPDATE_SERVER_MESSAGE = 1000;

common/pb/card_attributes.proto

-syntax = "proto2";
 enum CardAttribute {
     AttrTapped = 1;
     AttrAttacking = 2;

common/pb/color.proto

-syntax = "proto2";
 message color {
     optional uint32 r = 1;
     optional uint32 g = 2;
     optional uint32 b = 3;
     optional uint32 a = 4;
-}
+}
\ No newline at end of file

common/pb/command_attach_card.proto

-syntax = "proto2";
 import "game_commands.proto";
 message Command_AttachCard {
     extend GameCommand {

common/pb/command_change_zone_properties.proto

-syntax = "proto2";
 import "game_commands.proto";
 
 message Command_ChangeZoneProperties {

common/pb/command_concede.proto

-syntax = "proto2";
 import "game_commands.proto";
 message Command_Concede {
     extend GameCommand {

common/pb/command_create_arrow.proto

-syntax = "proto2";
 import "game_commands.proto";
 import "color.proto";
 

common/pb/command_create_counter.proto

-syntax = "proto2";
 import "game_commands.proto";
 import "color.proto";
 

common/pb/command_create_token.proto

-syntax = "proto2";
 import "game_commands.proto";
 message Command_CreateToken {
     extend GameCommand {

common/pb/command_deck_del.proto

-syntax = "proto2";
 import "session_commands.proto";
 
 message Command_DeckDel {

common/pb/command_deck_del_dir.proto

-syntax = "proto2";
 import "session_commands.proto";
 
 message Command_DeckDelDir {

common/pb/command_deck_download.proto

-syntax = "proto2";
 import "session_commands.proto";
 
 message Command_DeckDownload {

common/pb/command_deck_list.proto

-syntax = "proto2";
 import "session_commands.proto";
 
 message Command_DeckList {

common/pb/command_deck_new_dir.proto

-syntax = "proto2";
 import "session_commands.proto";
 
 message Command_DeckNewDir {
@@ -8,4 +7,4 @@ message Command_DeckNewDir {
     optional string path = 1;
     optional string dir_name = 2;
 }
- 
+ 
\ No newline at end of file

common/pb/command_deck_select.proto

-syntax = "proto2";
 import "game_commands.proto";
 message Command_DeckSelect {
     extend GameCommand {

common/pb/command_deck_upload.proto

-syntax = "proto2";
 import "session_commands.proto";
 
 message Command_DeckUpload {

common/pb/command_del_counter.proto

-syntax = "proto2";
 import "game_commands.proto";
 message Command_DelCounter {
     extend GameCommand {

common/pb/command_delete_arrow.proto

-syntax = "proto2";
 import "game_commands.proto";
 message Command_DeleteArrow {
     extend GameCommand {

common/pb/command_draw_cards.proto

-syntax = "proto2";
 import "game_commands.proto";
 message Command_DrawCards {
     extend GameCommand {

common/pb/command_dump_zone.proto

-syntax = "proto2";
 import "game_commands.proto";
 message Command_DumpZone {
     extend GameCommand {