makefile: remove unused SGI_ABI variable (GH-29325) (GH-29368)

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] https://github.com/python/cpython/pull/3294

Signed-off-by: Filipe Laíns <lains@riseup.net>
(cherry picked from commit 38982ab13629a0802ea721a5917da0b98d0d5a0a)

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