Revert "fix: enable on-demand leader schedule computation in get_slot…

[KirillLykov]

The original change was to fix the problem with get_slot_leaders.

After looking deeper into it, I agree that the fix was incorrect, the test is incorrect, and, hence, the issue requires additional investigation.

This reverts commit ce1e9b3.

[mergify]

If this PR represents a change to the public RPC API:

1. Make sure it includes a complementary update to rpc-client/ (example)
2. Open a follow-up PR to update the JavaScript client @solana/kit (example)

Thank you for keeping the RPC clients in sync with the server API @KirillLykov.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.1%. Comparing base (43b06cb) to head (1d33386).
⚠️ Report is 2282 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7917   +/-   ##
=======================================
  Coverage    83.1%    83.1%           
=======================================
  Files         810      810           
  Lines      357414   357386   -28     
=======================================
- Hits       297133   297114   -19     
+ Misses      60281    60272    -9     

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