Remove unused stats in PQFastScan

[chasingegg]

Remove unused stats in PQFastScan, where milvus could fail on global variable initialization on an sse4_2 machine.

[sre-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: chasingegg

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [chasingegg]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[chasingegg]

/kind improvement

[alexanderguzhva]

@chasingegg Could you please elaborate why the failure can happen? Thanks

[chasingegg]

@chasingegg Could you please elaborate why the failure can happen? Thanks

We will get _GLOBAL__sub_I_IndexPQFastScan.cpp on sse4_2, which means the initialization code for a global object or function in the file caused the problem, that is FastScan_stats. By removing this, the problem disappears.

[alexanderguzhva]

@chasingegg Could you please elaborate why the failure can happen? Thanks

We will get _GLOBAL__sub_I_IndexPQFastScan.cpp on sse4_2, which means the initialization code for a global object or function in the file caused the problem, that is FastScan_stats. By removing this, the problem disappears.

Also, as far I as remember, some of the veeeeeery recent commits to Faiss moved this stats from global to local variable, which is passed in a function. We can go this way as well.

[alexanderguzhva]

/lgtm