buildman: Drop unused OUTPUT_FILE constant

This is not actually used but its presence suggests that it is the
filename for the board database. Drop it to avoid confusion.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/buildman/boards.py b/tools/buildman/boards.py
index 9e7b486..e7aa0d8 100644
--- a/tools/buildman/boards.py
+++ b/tools/buildman/boards.py
@@ -25,7 +25,6 @@
 from u_boot_pylib import tout
 
 ### constant variables ###
-OUTPUT_FILE = 'boards.cfg'
 CONFIG_DIR = 'configs'
 SLEEP_TIME = 0.03
 COMMENT_BLOCK = f'''#