Summary:
Remove the YB_BITMAP_DISCOURAGE_MODIFIER cost fudge factor that seems to be no longer necessary after multiple improvements to the YB cost model.

Jira: DB-12482

Test Plan:
./yb_build.sh release --java-test 'org.yb.pgsql.TestPgRegressYbBitmapPlan'
./yb_build.sh release --java-test 'org.yb.pgsql.TestPgRegressYbBitmapScans'
./yb_build.sh release --java-test 'org.yb.pgsql.TestPgRegressPlanner'

* TAQO cost-validation-bitmapscans regression report: https://taqo.dev.yugabyte.com/report/view/a6c6ea92196f31f48d909dd58022ee75?path=20250611-113645%2Findex_None.html
  * The maximum slow down compared to the baseline was 4 ms.
  * Top 5 queries had no plan changes
  * The 6th switched from Seq Scan to bitmap plan showing 14.08 vs. 18.19 ms difference.

Reviewers: telgersma, gkukreja, mihnea

Reviewed By: telgersma

Subscribers: jason, yql

Differential Revision: https://phorge.dev.yugabyte.com/D44148