makefile: remove unused SGI_ABI variable (pythonGH-29325)

SGI_ABI support was removed in [1] but this variable was never removed
from the makefile. Currently, it is just a bad variable that does not
get replaced by the configure script.

[1] python#3294

Signed-off-by: Filipe Laíns <lains@riseup.net>

Makefile.pre.in

@@ -104,7 +104,6 @@ PY_CPPFLAGS=	$(BASECPPFLAGS) -I. -I$(srcdir)/Include $(CONFIGURE_CPPFLAGS) $(CPP
 PY_LDFLAGS=	$(CONFIGURE_LDFLAGS) $(LDFLAGS)
 PY_LDFLAGS_NODIST=$(CONFIGURE_LDFLAGS_NODIST) $(LDFLAGS_NODIST)
 NO_AS_NEEDED=	@NO_AS_NEEDED@
-SGI_ABI=	@SGI_ABI@
 CCSHARED=	@CCSHARED@
 # LINKFORSHARED are the flags passed to the $(CC) command that links
 # the python executable -- this is only needed for a few systems