[dependabot] remove invalid if property from github-actions update (#13402)

Dependabot configuration files (.github/dependabot.yml) do not support
conditional execution or workflow expression syntax such as the `if:`
property. Including `if:` causes a schema validation error when
Dependabot parses the file:
"The property '#/updates/0/' contains additional properties ["if"]
outside of the schema when none are allowed".

This commit removes `if: github.repository == "openthread/openthread"`
from the github-actions package ecosystem update configuration to
conform to Dependabot's v2 specification.

Because GitHub automatically disables Dependabot version updates and
automated pull requests on repository forks by default (unless
explicitly enabled by the fork owner in repository settings), removing
this condition resolves the schema validation error without generating
unwanted pull requests on standard forks.

.github/dependabot.yml

@@ -29,7 +29,6 @@
 version: 2
 updates:
   - package-ecosystem: "github-actions"
-    if: github.repository == "openthread/openthread"
     directory: "/"
     schedule:
       interval: "weekly"