Remove CvdFlag

This has no more users.

Bug: b/389804379
Test: /usr/bin/bazel test '//...'

base/cvd/cuttlefish/host/commands/cvd/BUILD.bazel

@@ -89,7 +89,6 @@ cc_library(
         "cli/commands/stop.cpp",
         "cli/commands/try_acloud.cpp",
         "cli/commands/version.cpp",
-        "cli/flag.cpp",
         "cli/frontline_parser.cpp",
         "cli/interruptible_terminal.cpp",
         "cli/parser/cf_configs_common.cpp",
@@ -185,7 +184,6 @@ cc_library(
         "cli/commands/stop.h",
         "cli/commands/try_acloud.h",
         "cli/commands/version.h",
-        "cli/flag.h",
         "cli/frontline_parser.h",
         "cli/interruptible_terminal.h",
         "cli/parser/cf_configs_common.h",
@@ -316,9 +314,6 @@ cc_test(
         "unittests/selector/client_lexer_helper.cpp",
         "unittests/selector/client_lexer_helper.h",
         "unittests/selector/client_lexer_test.cpp",
-        "unittests/selector/cvd_flags_helper.cpp",
-        "unittests/selector/cvd_flags_helper.h",
-        "unittests/selector/cvd_flags_test.cpp",
         "unittests/selector/group_record_test.cpp",
         "unittests/selector/host_tool_target_test.cpp",
         "unittests/selector/instance_database_helper.cpp",

base/cvd/cuttlefish/host/commands/cvd/cli/commands/restart.cpp

@@ -25,7 +25,6 @@
 #include "common/libs/utils/flag_parser.h"
 #include "common/libs/utils/result.h"
 #include "host/commands/cvd/cli/commands/command_handler.h"
-#include "host/commands/cvd/cli/flag.h"
 #include "host/commands/cvd/cli/selector/selector.h"
 #include "host/commands/cvd/cli/types.h"
 #include "host/commands/cvd/cli/utils.h"