remove unnecessary files

.github/release-drafter-dev-6.1.x.yml

@@ -1,7 +0,0 @@
-_extends: growi:.github/release-drafter.yml
-
-prerelease: true
-
-# Filter previous releases to consider only those with the tags starts with 'v6.1'
-include-pre-releases: true
-tag-prefix: v6.1

.github/release-drafter-dev-7.0.x.yml

@@ -1,7 +0,0 @@
-_extends: growi:.github/release-drafter.yml
-
-prerelease: true
-
-# Filter previous releases to consider only those with the tags starts with 'v7.0'
-include-pre-releases: true
-tag-prefix: v7.0