innobase_end(): Do not attempt to shrink the system tablespace if
innodb_read_only=ON or innodb_force_recovery>4. This fixes a regression
due to commit 2d6c2f22 (MDEV-32452).

This bug was caught when testing a fix of MDEV-34200, which adds
SET GLOBAL innodb_fast_shutdown=0 to the test innodb.undo_upgrade.