make: remove leftover help flag

This flag is gone since we move to go modules.
Signed-off-by: Sébastien Han <seb@redhat.com>

Makefile

@@ -188,7 +188,6 @@ Targets:
     test-integration   Runs integration tests.
     mod.check          Check / Tidy current modules.
     mod.update         Update all modules.
-    vendor.check       Checks if vendor dependencies changed.
     vet                Runs lint checks on go sources.
 
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