It was somewhat broken. The order of evaluation of function arguments is
implementation-specific, so in a statement like:

ereport(ERROR,
        (errcode(ERRCODE_INTERNAL_ERROR),
	 errOmitLocation(true)));

We cannot assume that errOmitLocation() is evaluated after errcode(). If
errOmitLocation() is evaluated first, the errcode() call could overwrite
the omit_location field, seeing that the error code was "internal error".

Almost all of the errOmitLocation calls in the codebase were superfluous
anyway. The default logic is to omit the location for anything else than
ERRCODE_INTERNAL error, so it is not necessary to call errOmitLocation(true),
if errcode (other than ERRCODE_INTERNAL_ERROR) is given. Likewise
errOmitLocation(false) is not needed for internal errors.

Remove the whole errOmitLocation() function. It's not really needed. The
most notable callsite where it mattered was in cdbdisp.c, but that one
was broken by the order-of-evaluation issue. Use a different error code
there. What we really should do there is to pass the error code from the
segment back to the client, but I'll leave that for another day.