Revert Windows CI BF16 disable flags from PR #6682

[Copilot]

This PR removes the -DNCNN_BF16=OFF overrides introduced in commit b2fd28c08282d16297b34d251fa99ef0cef273cc and restores the Windows workflow CMake command lines to their pre-#6682 state. Scope is intentionally limited to the five affected invocations in .github/workflows/windows.yml.

• Workflow reversion (.github/workflows/windows.yml)

  • Restored original CMake args for:
    • protobuf build step
    • x64
    • x64-sse2
    • x64-avx
    • x86
  • No other workflow/jobs/options were modified.

• Exact change pattern

  • Removed only the BF16 override argument from each affected command.

# before (from #6682)
cmake ... -DNCNN_BUILD_TESTS=ON -DNCNN_BF16=OFF ..

# after (restored)
cmake ... -DNCNN_BUILD_TESTS=ON ..

Original prompt

Revert the changes introduced by commit b2fd28c08282d16297b34d251fa99ef0cef273cc from pull request #6682 in Tencent/ncnn.

Scope:

• File affected: .github/workflows/windows.yml
• Revert the addition of -DNCNN_BF16=OFF that was added to the Windows CI CMake invocations in this commit.
• Restore the previous command lines exactly as they were before this commit in all affected workflow steps.

Affected hunks to revert:

1. protobuf build cmake invocation near line 104
2. x64 build cmake invocation near line 139
3. x64-sse2 build cmake invocation near line 151
4. x64-avx build cmake invocation near line 159
5. x86 build cmake invocation near line 167

Goal:

• Create a pull request that reverts only these workflow changes from the specified commit, without altering unrelated code.
• Keep formatting and YAML structure intact.

The following is the prior conversation context from the user's chat exploration (may be truncated):

User: revert changes in @Tencent/ncnn/pull/6682/changes/b2fd28c08282d16297b34d251fa99ef0cef273cc

Assistant: I’m waiting for your response to the confirmation dialog before I can continue.

This pull request was created from Copilot chat.

[tencent-adm]

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