Remove leftovers of IRIX port

This removes the remaining pieces of the IRIX port that was removed by
ea91a6be89.

src/makefiles/Makefile.irix

@@ -1,19 +0,0 @@
-AROPT = crs
-rpath = -Wl,-rpath,'$(rpathdir)'
-
-DLSUFFIX = .so
-# PIC is default
-CFLAGS_SL =
-
-override CPPFLAGS += -U_NO_XOPEN4
-
-ifneq ($(GCC), yes)
-  CFLAGS += -woff 1164,1171,1185,1195,1552
-endif
-LDFLAGS += -Wl,-woff,15 -Wl,-woff,84
-
-# Rule for building a shared library from a single .o file
-%.so: %.o
-	$(CC) $(CFLAGS) $(LDFLAGS) $(LDFLAGS_SL) -shared -o $@ $<
-
-sqlmansect = 5sql