Makefile: drop -Wno-universal-initializer from SP_EXTRA_FLAGS

Makefile: drop -Wno-universal-initializer from SP_EXTRA_FLAGS

Commit 1c96642326 ("sparse: allow '{ 0 }' to be used without warnings",
2020-05-22) added -Wno-universal-initializer to the SP_EXTRA_FLAGS in
order to suppress potential sparse warnings from using '{0}' as an
aggregate initializer. At that time, the default was for sparse to
issue warnings (i.e. the default was -Wuniversal-initializer) if such
an initializer was used to initialize an aggregate whose first member
was a pointer type. However, this default was changed just a few days
later to -Wno-universal-initializer (first released in sparse v0.6.2)
and has been so in all subsequent release versions of sparse.  Thus,
including -Wno-universal-initializer in the SP_EXTRA_FLAGS variable is
redundant.

Remove the unnecessary warning flag from SP_EXTRA_FLAGS, essentially
reverting commit 1c96642326.

Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>