ExternalData: Drop 'not security software' disclaimer (#15060)

Let clients do their own research on the current strength of each
hash algorithm to choose what is best for their needs.

Suggested-by: Nico Schlömer <[email protected]>

Modules/ExternalData.cmake

@@ -76,7 +76,7 @@
 #  SHA512      .sha512   US Secure Hash Algorithms, RFC 4634
 #
 # Note that the hashes are used only for unique data identification and
-# download verification.  This is not security software.
+# download verification.
 #
 # Example usage:
 #