Summary:
4fc50fbfe6b7 / D38825 added some code to `yb_get_ybctid_width` that opened the relcache entry for an index but never closed it. This would lead to the following warning:

```
WARNING:  relcache reference leak: relation "t_expr_idx" not closed
```

This diff closes the relcache entry after we're done using it.
Jira: DB-13923

Test Plan:
```
./yb_build.sh release --java-test org.yb.pgsql.TestPgEstimatedDocdbResultWidth#testNoRelcacheReferenceLeakWithExpressionIndex
```

Reviewers: gkukreja

Reviewed By: gkukreja

Subscribers: patnaik.balivada, yql

Differential Revision: https://phorge.dev.yugabyte.com/D42843