processDiagnosticMessages.ts: linted, removed unused code (#18697)
processDiagnosticMessages.ts: linted, removed unused code (#18697)

Added following line to generated output:
"// generated from 'src/diagnosticMessages.json' by 'scripts/processDiagnosticMessages.ts'\r\n" +

Fixes https://github.com/Microsoft/TypeScript/issues/3591
microsoft/TypeScript,microsoft/TypeScript,minestarks/TypeScript,SaschaNaz/TypeScript,alexeagle/TypeScript,weswigham/TypeScript,donaldpipowitch/TypeScript,alexeagle/TypeScript,nojvek/TypeScript,basarat/TypeScript,minestarks/TypeScript,kitsonk/TypeScript,nojvek/TypeScript,kitsonk/TypeScript,weswigham/TypeScript,kitsonk/TypeScript,RyanCavanaugh/TypeScript,alexeagle/TypeScript,Microsoft/TypeScript,basarat/TypeScript,RyanCavanaugh/TypeScript,Microsoft/TypeScript,SaschaNaz/TypeScript,donaldpipowitch/TypeScript,nojvek/TypeScript,minestarks/TypeScript,basarat/TypeScript,kpreisser/TypeScript,RyanCavanaugh/TypeScript,microsoft/TypeScript,SaschaNaz/TypeScript,donaldpipowitch/TypeScript,nojvek/TypeScript,kpreisser/TypeScript,kpreisser/TypeScript,SaschaNaz/TypeScript,weswigham/TypeScript,basarat/TypeScript,donaldpipowitch/TypeScript,Microsoft/TypeScript