chore: [java] synthtool not to create an initial gitignore. (#1672) (#5590)

* chore: [java] synthtool not to create an initial gitignore.

Given that new generated libraries will come to the Java monorepo
going forward and that any new hand-written libraries can get
a new .gitignore from humans, there's no need to automatically
prepare .gitignore file.

* deleting gitignore

* deleting .gitignore from golden
Source-Link: https://togithub.com/googleapis/synthtool/commit/0baf7e4b652fcc7bf77ec341965c62c0b67e722d
Post-Processor: gcr.io/cloud-devrel-public-resources/owlbot-java:latest@sha256:25a679b48797fcdd98fe339c885d9595a8be5c16a275d1b466ba98e6951a3217

Author: gcf-owl-bot[bot]