fix: Revert "feat(gradle): use maven datasource (#3062)"

lib/config/definitions.js

lib/manager/gradle/gradle-updates-report.js

lib/manager/gradle/index.js

test/_fixtures/gradle/MultiProjectUpdatesReport.json

test/_fixtures/gradle/updatesReport.json

test/_fixtures/gradle/updatesReportEmpty.json

test/datasource/maven.spec.js

test/manager/gradle/__snapshots__/index.spec.js.snap

test/manager/gradle/index.spec.js