refactor: Simplify targets

[tony]

Eliminate redundant targets / window_index's across codebase.

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 88.75%. Comparing base (9eac8b0) to head (22b51e8).

Files	Patch %	Lines
src/libtmux/session.py	0.00%	3 Missing ⚠️
src/libtmux/window.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #536   +/-   ##
=======================================
  Coverage   88.75%   88.75%           
=======================================
  Files          37       37           
  Lines        3965     3965           
  Branches      574      574           
=======================================
  Hits         3519     3519           
  Misses        304      304           
  Partials      142      142           

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