refactor: drop redundant database index

[krille-chan]

The k_index we created here was actually never used and just duplicated the key which blows up the database without any benefit. We can safely drop it to decrease database size.#

Have tested it on macOS native and FluffyChat on Android and everything still seems to work fine. But it also doesn't decrease the database size actually. However we still don't need this index so we should remove it

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 60.38%. Comparing base (474c4d9) to head (827cf91).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2476   +/-   ##
=======================================
  Coverage   60.38%   60.38%           
=======================================
  Files         162      162           
  Lines       20564    20564           
=======================================
  Hits        12418    12418           
  Misses       8146     8146           

Files with missing lines	Coverage Δ
lib/src/database/sqflite_box.dart	81.25% <100.00%> (ø)

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