Summary:
Regress test yb_pg_pg_trgm has workarounds to explicitly set
enable_bitmapscan on because the default in upstream postgres is "on"
while the default in YB was "off" as of commit
b038851f41ff026cb11fe00179702d823a311fdc.  Then came commit
58771875a40e892c99d7663aadf4ab33ce797b27 which changed the default back
to "on".

So remove the enable_bitmapscan settings that were previously added.

Jira: DB-15360

Test Plan:
On Almalinux 8:

    ./yb_build.sh fastdebug --gcc11 --java-test TestPgRegressContribPgTrgm

Close: #26030

Reviewers: telgersma

Reviewed By: telgersma

Subscribers: yql

Differential Revision: https://phorge.dev.yugabyte.com/D41825