chore: [java] remove flatten plugin dependency check (#1663) (#2124)

chore: remove check for flatten plugin

We have had the check for the flatten-maven-plugin in each
Cloud Java client repository. However, the behavior of the plugin
has been stable and its not each repository's responsibility to
assert the plugin's behavior. A new check is going to be added at
the googleapis/java-shared-config repository to assert the
plugin's behavior when we upgrade its version.
Source-Link: https://togithub.com/googleapis/synthtool/commit/9266ddc3b17fc15f34d2fb88ce8c5f1a4bfe64b0
Post-Processor: gcr.io/cloud-devrel-public-resources/owlbot-java:latest@sha256:ae72564aa9c368b9ccd96f4af21f87889fd83b9e60635b80844deb5a2ccd08aa

Author: gcf-owl-bot[bot]

.github/.OwlBot.lock.yaml

@@ -13,4 +13,4 @@
 # limitations under the License.
 docker:
   image: gcr.io/cloud-devrel-public-resources/owlbot-java:latest
-  digest: sha256:264c6d5da60ff1684fbdd2b268d6a3ffca2038246e0948a06f15ca0c3cf28ce8
+  digest: sha256:ae72564aa9c368b9ccd96f4af21f87889fd83b9e60635b80844deb5a2ccd08aa