Convert 12 word seed with passphrase -> 12 word seed without passphrase
No, this is not possible.
The mnemonic and passphrase are put through PBKDF2. Any change in the input results in a completely different output.
While it is possible that there exists some 12 word mnemonic that just happens to hash to the same seed as your original mnemonic + passphrase, this would require bruteforcing all possible mnemonics to discover, and would not be derivable from the original mnemonic in any form - a hash collision would be completely unrelated.