fix: use physical index to removePair

[AsterDY]

fix #600

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 71.42857% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 77.58%. Comparing base (eb25ba2) to head (456f13d).

Files	Patch %	Lines
ast/node.go	71.42%	1 Missing and 1 partial ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #601      +/-   ##
==========================================
+ Coverage   73.80%   77.58%   +3.77%     
==========================================
  Files          62       83      +21     
  Lines        8266    11538    +3272     
==========================================
+ Hits         6101     8952    +2851     
- Misses       1826     2188     +362     
- Partials      339      398      +59     

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