Summary:
By mistake, the LogReader memory limit was applied to CDC as well as xCluster.

This was intended to only be turned on after more CDC changes (see [#28779: CDCSDK Add limit for log reader memory used by CDC](https://github.com/yugabyte/yugabyte-db/issues/28779)).

The bad code is in ReadFromLogCacheForXRepl, where it always applies limit instead of using the obey_memory_limit parameter passed in.

Fixes #29066
Jira: DB-18830

Test Plan:
The following test reproduces the failure and now passes:
```
ybd release --cxx-test cdcsdk_consistent_stream-test --gtest_filter '*.TestCDCSDKConsistentStreamWithManyTransactions' --test-args ' --read_wal_memory_bytes=10'
```

Reviewers: xCluster, hsunder

Reviewed By: hsunder

Subscribers: ybase

Differential Revision: https://phorge.dev.yugabyte.com/D47728