binman: Remove redundant SetAllowFakeBlob from blob-ext entry

Entry_blob_ext contains an implementation of SetAllowFakeBlob that is
identical to the one in the base Entry class, remove it.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/etype/blob_ext.py b/tools/binman/etype/blob_ext.py
index fba6271..d6b0ca1 100644
--- a/tools/binman/etype/blob_ext.py
+++ b/tools/binman/etype/blob_ext.py
@@ -26,11 +26,3 @@
     def __init__(self, section, etype, node):
         Entry_blob.__init__(self, section, etype, node)
         self.external = True
-
-    def SetAllowFakeBlob(self, allow_fake):
-        """Set whether the entry allows to create a fake blob
-
-        Args:
-            allow_fake_blob: True if allowed, False if not allowed
-        """
-        self.allow_fake = allow_fake