Delete the custom gitignore file (#72019)

* Delete the custom gitignore file

The root gitignore file already contains most of these settings and should be preferred.

* Delete .gitignore

src/libraries/System.Runtime.InteropServices.JavaScript/gen/.gitignore

-.vs/
-**/bin
-**/obj
-*.binlog
-BenchmarkDotNet.Artifacts
\ No newline at end of file

src/libraries/System.Runtime.InteropServices/gen/.gitignore

-.vs/
-**/bin
-**/obj
-*.binlog
-BenchmarkDotNet.Artifacts
\ No newline at end of file