Remove separate-pull-requests workaround after upstream release-please fix #1903

Context

PR #1902 added "separate-pull-requests": false to the release-please config as a
workaround for a bug where pull-request-footer is ignored for Java snapshot PRs in
single-package repos.

Upstream fix: googleapis/release-please#2685

Action

Once the upstream fix is merged and released, remove "separate-pull-requests": false
from .github/config/release-please-config.json.