Revert "Enable Coverity "streams""

It didn't work. Coverity classifies all our builds as belonging to the
stream called 'dosbox-staging' and there's no option to change it.

This reverts commit e86e6e353e.

.github/workflows/coverity.yml

@@ -50,7 +50,6 @@ jobs:
           --form token="${COVERITY_TOKEN}" \
           --form email="${COVERITY_EMAIL}" \
           --form file=@package.tar.gz \
-          --form version="${GITHUB_SHA}" \
+          --form version="${GITHUB_REF}" \
           --form description="${GITHUB_REPOSITORY}" \
-          --form stream="${GITHUB_REF}" \
           "https://scan.coverity.com/builds?project=dosbox-staging"