fix #268: Do not build :latest for postfix-exporter from main branch

As we are now aligning the build version of `postfix-exporter` with the
build version of the image.

Hance it doesn't make sense to build `:latest` from the main branch
anymore, but to build it from the latest tag (as it is the case with the
main docker image).

Author: bokysan

.github/workflows/master.yml

@@ -52,7 +52,6 @@ jobs:
         with:
           DOCKER_ACCESS_TOKEN: '${{ secrets.DOCKER_ACCESS_TOKEN }}'
           tags: |
-            boky/postfix-exporter:latest
             boky/postfix-exporter:edge
 
   Build_Alpine: