how to restore default configurations for dovecot

If you didn't make a backup, and you don't want to re-install the dovecot packages, then there isn't an easy way to simply revert your configuration.

You aren't entirely clear about your no reinstall requirement. Do you mean you don't want to reinstall the system or anything at all. You could purge and reinstall the dovecot packages without reinstalling the entire system.

The version of dovecot I am using also leaves a clean copy of the default config files in /usr/share/dovecot/. You could try restoring those files.

Past that, you could just download the .deb files, if they are not in your apt cache (/var/cache/apt/archives/), extract the archive(s) and manually restore the default configuration files, or anything else you changed.

mkdir /tmp/blah ; cd /tmp/blah
ar vx /var/cache/apt/archives/dovecot-common_1%3a1.2.15-7_amd64.deb
tar -xzvf data.tar.gz
# see all the nice default config files from the common package
find etc/