box: remove useless region_truncate from box_{space,index}_id_by_name

region_truncate() is not required there as box_index_get() cleans up
fiber()->gc anyway - see txn_commit_ro_stmt(). Actually, it's even
dangerous: if fiber_gc() called by box_index_get() happens to free
memory, region_truncate() will crash on the assertion:

  region_truncate: Assertion `region_used(region) >= used' failed.