feat(StoneDB 8.0): refactor ConditionNumberFromMultipleEquality function. (#583)

[lujiashun]

[summary]
1 improve the function's readability;
2 remove "stonedb8" comments;

Summary about this PR

Issue Number: close #583

[summary]
function ConditionNumberFromMultipleEquality in the file storage/tianmu/core/query.cpp
the reason for the 8.0 adaptor for this function is :Item_equal_iterator is deleted from mysql8.0.
Reference:mysql/mysql-server@908666c

  List_STL_Iterator<Item_field> ifield;
  List_STL_Iterator<Item_field> li;
  li = conds->get_fields().begin();

the logic is correct, but code can be cleaner.

1 improve the function's readability;
2 remove "stonedb8" comments;

Tests Check List

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Changelog

• New Feature
• Bug Fix
• Improvement
• Performance Improvement
• Build/Testing/CI/CD
• Documentation
• Not for changelog (changelog entry is not required)

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features

[mergify]

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[RingsC]

LGTM

[DandreChen]

LGTM