refactor(benchpress): remove two mutable exports from largetable/util.ts (#32425)

Mutable exports, i.e. using this pattern

```
export let x = 0;
export function f() {
  x += 1;
}
```

is problematic to transpile to CommonJS and Goog.module systems, we are
working on banning it google internal codebase.

The workaround is adding an explcit getter function.

PR Close #32425

modules/benchmarks/src/largetable/util.ts

@@ -13,8 +13,8 @@ export class TableCell {
 }
 
 let tableCreateCount: number;
-export let maxRow: number;
-export let maxCol: number;
+let maxRow: number;
+let maxCol: number;
 let numberData: TableCell[][];
 let charData: TableCell[][];