[skip changelog] Revert "Add skip changelog prefix to Dependabot workflow update commits (#1381)" (#1389)

This reverts commit 4fbf7b7.

It turns out there is an undocumented maximum length of 15 characters for this prefix:

https://github.com/arduino/arduino-cli/network/updates

```
Dependabot encountered the following error when parsing your .github/dependabot.yml:

The property '#/updates/0/commit-message/prefix' was not of a maximum string length of 15
Please update the config file to conform with Dependabot's specification.
```
Unfortunately, even though it is possible to reduce the prefix length by 1 with the removal of the trailing space, we
still end up with 16 characters.

Author: per1234

.github/dependabot.yml

@@ -9,7 +9,5 @@ updates:
     directory: / # Check the repository's workflows under /.github/workflows/
     schedule:
       interval: daily
-    commit-message:
-      prefix: "[skip changelog] "
     labels:
       - "topic: dependencies"