Unnecessary/confusing TO_LAB_UNUSED

[skliper]

Is your feature request related to a problem? Please describe.
The only two places TO_LAB_UNUSED is referenced:

to_lab/fsw/src/to_lab_app.h

Line 39 in 81d85da

#define TO_LAB_UNUSED CFE_SB_MSGID_RESERVED

and

to_lab/fsw/src/to_lab_app.c

Lines 205 to 209 in 81d85da

	if (!CFE_SB_IsValidMsgId(TO_LAB_Subs->Subs[i].Stream))
	{
	/* Only process until invalid MsgId (aka TO_LAB_UNUSED) is found*/
	break;
	}

Since the test is for a valid msgid, it can only be CFE_SB_MSGID_RESERVED.

Describe the solution you'd like
Remove TO_LAB_UNUSED

Describe alternatives you've considered
None

Additional context
Old custom to subs table file was using it, but since it's in to_lab_app.h it's not really exposed. Just remove to avoid this potential confusion in the future.

Requester Info
Jacob Hageman - NASA/GSFC