config: remove xyz_gapic.yaml files

Currently we have [78 xyz_gapic.yaml files](https://github.com/search?q=repo%3Agoogleapis%2Fgoogleapis+path%3A*_gapic.yaml&type=code) in the googleapis repo, where xyz is the service name.

These contains a mixture of language-neutral and language-specific settings, although currently they're only consumed (I believe) by the PHP and Java generators.

All this configuration should be in the "right" place as determined by #3040.

Start by reviewing the 78 *_gapic.yaml files linked in the issue and the configuration placement proposed in #3040. Check how the PHP and Java generators consume these files, then verify that removing them leaves configuration in the intended location and that generation still works.

Written by the indexing model from the issue text.