Fix LilyGo_TLora_V2_1_1_6_terminal_chat build

[3dpgg]

This change addresses two issues. The first is that the LilyGo_TLora_V2_1_1_6_terminal_chat build would try to compile simple_repeater/MyMesh.cpp. All other examples of terminal chat targets are instead building simple_secure_chat/main.cpp . This change would align this build to the rest of the builds.

The second issue, found during the course of investigating the first, stems from simple_repeater/MyMesh.cpp using the MAX_NEIGHBOURS #define to control whether the neighbor list is kept. Repeaters that keep this list must define this value, and if the value is not defined, then all neighbor-related functionality is compiled out. However, the code that replies to
REQ_TYPE_GET_NEIGHBOURS did not properly check for this #define, and thus any target that compiles simple_repeater/MyMesh.cpp without defining MAX_NEIGHBOURS would get an undefined variable compilation error.

As a practical matter though, there are no targets that compile simple_repeater/MyMesh.cpp AND do not define MAX_NEIGHBOURS, except this build due to the first issue. As a result, the second issue is addressed only as a matter of completeness. The expected behavior with this change is that such a repeater would send a valid reply indicating zero known neighbors.