Remove compatibility declarations for InitMaterializedSRF()

These routines have been renamed in a19e5ce.  There is no need to keep
the compatibility declarations on HEAD, as once an extension moves to
the new routine name when compiling with v16~ the code would work the
same way when recompiled on v15.  No backpatch to v15 for this one,
because ABI compatibility has to be maintained there.

Discussion: https://postgr.es/m/20221013194820.ciktb2sbbpw7cljm@awork3.anarazel.de

src/backend/utils/fmgr/funcapi.c

@@ -56,15 +56,6 @@ static bool resolve_polymorphic_tupdesc(TupleDesc tupdesc,
 static TypeFuncClass get_type_func_class(Oid typid, Oid *base_typeid);
 
 
-/*
- * Compatibility function for v15.
- */
-void
-SetSingleFuncCall(FunctionCallInfo fcinfo, bits32 flags)
-{
-	InitMaterializedSRF(fcinfo, flags);
-}
-
 /*
  * InitMaterializedSRF
  *

src/include/funcapi.h

@@ -297,11 +297,6 @@ HeapTupleGetDatum(const HeapTupleData *tuple)
 											 * BlessTupleDesc(). */
 extern void InitMaterializedSRF(FunctionCallInfo fcinfo, bits32 flags);
 
-/* Compatibility declarations, for v15 */
-#define SRF_SINGLE_USE_EXPECTED MAT_SRF_USE_EXPECTED_DESC
-#define SRF_SINGLE_BLESS		MAT_SRF_BLESS
-extern void SetSingleFuncCall(FunctionCallInfo fcinfo, bits32 flags);
-
 extern FuncCallContext *init_MultiFuncCall(PG_FUNCTION_ARGS);
 extern FuncCallContext *per_MultiFuncCall(PG_FUNCTION_ARGS);
 extern void end_MultiFuncCall(PG_FUNCTION_ARGS, FuncCallContext *funcctx);