makefile: remove unused SGI_ABI variable (GH-29325) (GH-29370)

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] #3294

Signed-off-by: Filipe Laíns <lains@riseup.net>
(cherry picked from commit 38982ab)

Co-authored-by: Filipe Laíns <lains@riseup.net>

Author: miss-islington

Makefile.pre.in

@@ -103,7 +103,6 @@ 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