config.mak.uname: remove obsolete SPARSE_FLAGS setting

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>