There is currently no feature in WinZip that can remove encryption from a Zip file.
In the Zip file format, the data itself in the zip file is encrypted.
AES Encryption Information
The Advanced Encryption Standard (AES) is a symmetric block cipher chosen by the U.S. government to protect classified information. AES is implemented in software and hardware throughout the world to encrypt sensitive data. It is essential for government computer security, cybersecurity, and electronic data protection.
Encryption is applied only to the content of files. It is performed after compression and not to any other associated data. The file data is encrypted byte-for-byte..
To remove encryption from the files that have been zipped, they would need to be extracted using the correct password. WinZip uses the password in combination with the appropriate key to decrypt the zipped data and recreate exact byte-for-byte duplicates of the originals. Once the files have been extracted, you can create a new Zip file without applying encryption. Afterward, you can delete the encrypted Zip file, leaving only the new, unencrypted Zip file.
You may find more information here.