Remove Int8GetDatum function

We have no uses of Int8GetDatum in our tree and did not have for a
long time (or never), and the inverse does not exist either.

Author: Kirill Reshke <reshkekirill@gmail.com>
Suggested-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/CALdSSPhFyb9qLSHee73XtZm1CBWJNo9+JzFNf-zUEWCRW5yEiQ@mail.gmail.com

src/include/postgres.h

@@ -134,16 +134,6 @@ CharGetDatum(char X)
 	return (Datum) X;
 }
 
-/*
- * Int8GetDatum
- *		Returns datum representation for an 8-bit integer.
- */
-static inline Datum
-Int8GetDatum(int8 X)
-{
-	return (Datum) X;
-}
-
 /*
  * DatumGetUInt8
  *		Returns 8-bit unsigned integer value of a datum.