releasetools: Remove the deprecated flag --board_config.

It has been long deprecated since commit
fdd8e69c42 (Donut).

Test: `m dist` with aosp_marlin-userdebug and aosp_angler-userdebug.
Test: No user of "--board_config" in code search.
Change-Id: Ic0336c22cac32de5de88161748d09b579f892e99

tools/releasetools/ota_from_target_files.py

@@ -21,9 +21,6 @@ a full OTA is produced.
 
 Usage:  ota_from_target_files [flags] input_target_files output_ota_package
 
-  --board_config  <file>
-      Deprecated.
-
   -k (--package_key) <key> Key to use to sign the package (default is
       the value of default_system_dev_certificate from the input
       target-files's META/misc_info.txt, or
@@ -1290,9 +1287,7 @@ def WriteABOTAPackageWithBrilloScript(target_file, output_file,
 def main(argv):
 
   def option_handler(o, a):
-    if o == "--board_config":
-      pass   # deprecated
-    elif o in ("-k", "--package_key"):
+    if o in ("-k", "--package_key"):
       OPTIONS.package_key = a
     elif o in ("-i", "--incremental_from"):
       OPTIONS.incremental_source = a
@@ -1354,7 +1349,6 @@ def main(argv):
   args = common.ParseOptions(argv, __doc__,
                              extra_opts="b:k:i:d:we:t:2o:",
                              extra_long_opts=[
-                                 "board_config=",
                                  "package_key=",
                                  "incremental_from=",
                                  "full_radio",