config.mak.uname: remove obsolete SPARSE_FLAGS setting

An upcoming commit will change the semantics of the SPARSE_FLAGS variable from an internal to a user only customisation variable. The MinGW configuration section contains an obsolete setting for this variable which was used (some years ago) to cater to an error in the Win32 system header files. Since 'sparse' does not currently support the MinGW platform, nobody on that platform can be relying on this setting today. Remove this use of the SPARSE_FLAGS variable. Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com> Acked-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>