wrapper.c: remove unused gitmkstemps() function

wrapper.c: remove unused gitmkstemps() function

The last call to the mkstemps() function was removed in commit 659488326
("wrapper.c: delete dead function git_mkstemps()", 22-04-2016). In order
to support platforms without mkstemps(), this functionality was provided,
along with a Makefile build variable (NO_MKSTEMPS), by the gitmkstemps()
function. Remove the dead code, along with the defunct build machinery.

Signed-off-by: Ramsay Jones <redacted>
Signed-off-by: Junio C Hamano <redacted>