[ObjC] Fix over-release in GPBTextFormatForMessage

When handling deeply nested unknown fields, the `subIndent` string was being over-released in `GPBTextFormatForMessage`, leading to crashes when generating text format for messages with deeply nested unknown fields.

The added test case reproduces this crash by creating a message with unknown fields nested deep enough to cause the indent string to be heap-allocated, and then adding siblings at that depth, triggering the incorrect release.

Removing the redundant `[subIndent release]` resolves the issue.

PiperOrigin-RevId: 895495564