[js/webgpu] Remove unnecessary initialization of var (#21312)

This var has been initialized to 0 in tint, so no need extra loop to do
it again:
```
  float tint_symbol_52[1][4] = (float[1][4])0;
  {
    for(int tint_symbol_53 = 0; (tint_symbol_53 < 1); tint_symbol_53 = (tint_symbol_53 + 1)) {
      {
        for(int tint_symbol_54 = 0; (tint_symbol_54 < 4); tint_symbol_54 = (tint_symbol_54 + 1)) {
          tint_symbol_52[min(uint(tint_symbol_53), 0u)][min(uint(tint_symbol_54), 3u)] = 0.0f;
        }
      }
    }
  }
```
### Description
<!-- Describe your changes. -->



### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->

js/web/lib/wasm/jsep/webgpu/ops/3rd-party/matmul_packed_webgpu.ts

@@ -328,13 +328,6 @@ fn main(@builtin(local_invocation_id) localId : vec3<u32>,
     var kStart = ${splitK ? `i32(globalId.z) * ${splitedDimInner}` : '0'};
 
     var acc : array<array<${type}, colPerThread>, rowPerThread>;
-
-    // Without this initialization strange values show up in acc.
-    for (var innerRow = 0; innerRow < rowPerThread; innerRow = innerRow + 1) {
-      for (var innerCol = 0; innerCol < colPerThread; innerCol = innerCol + 1) {
-        acc[innerRow][innerCol] = 0.0;
-      }
-    }
     ${matmulSnippet}
   }
 `;