Fix: redefined sm4 in pgcrypto and backend/crypto

[jiaqizho]

fix #ISSUE_Number

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Change logs

After the pgcrypto module supported sm4, the sm4-128-ofb mode was added in backend/sm4.c

This approach is very unclean. And the implementation of sm4 in pgcrypto is overwritten.

Why are the changes needed?

Does this PR introduce any user-facing change?

How was this patch tested?

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[my-ship-it]

LGTM