chore!(create-turbo): remove old pm arg (#8150)

### Description

We replaced this with an option instead of an arg, but cleaning up here!

### Testing Instructions

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packages/create-turbo/src/cli.ts

@@ -48,9 +48,6 @@ createTurboCli
   })
 
   .argument("[project-directory]")
-  // TODO: argument is still provided (but removed from help)
-  // for backwards compatibility, remove this in the next major
-  .argument("[package-manager]")
   .addOption(
     new Option(
       "-m, --package-manager <package-manager>",