refine prefer-range-scan behavior

[tisonkun]

Before, prefer-range-scan only removes the first full scan path if some range scan path exists. After the PR, prefer-range-scan removes all the full scan paths if some range scan path exists (TiFlash path or hint-indicated path will not be removed).
Also, it's a session only variable, cannot to be used as workaround since the workload may be ORM or other cases that a global variable is needed.

[tisonkun]

@xuyifangreeneyes you can describe what to refine here or ask for update the title. I'd prefer an expressive title instead of "refine" or "improve" xxx.