chore: Remove references to game-server module

[lqiu96]

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Error from the full-dependencies-check:

Error: ] Some problems were encountered while processing the POMs:
Error:  'dependencies.dependency.version' for com.google.cloud:google-cloud-game-servers:jar is missing. @ line 202, column 17
 @ 
Error:  The build could not read 1 project -> [Help 1]
Error:    
Error:    The project com.google.cloud:full-convergence-check:0.25.0 (/home/runner/work/java-cloud-bom/java-cloud-bom/tests/dependency-convergence/pom.xml) has 1 error
Error:      'dependencies.dependency.version' for com.google.cloud:google-cloud-game-servers:jar is missing. @ line 202, column [17](https://github.com/googleapis/java-cloud-bom/actions/runs/6645828051/job/18057979135?pr=6262#step:6:18)
Error:  
Error:  To see the full stack trace of the errors, re-run Maven with the -e switch.
Error:  Re-run Maven using the -X switch to enable full debug logging.
Error:  
Error:  For more information about the errors and possible solutions, please read the following articles:
Error:  [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/ProjectBuildingException
Error: Process completed with exit code 1.

from: https://github.com/googleapis/java-cloud-bom/actions/runs/6645828051/job/18057979135?pr=6262