exec_cmd: remove unused extern

This definition was added by commit 77cb17e9, but it's left unused since
commit 511707d4. Remove the left-over definition.
Signed-off-by: NErik Faye-Lund <kusmabite@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

exec_cmd.c

@@ -3,7 +3,6 @@
 #include "quote.h"
 #define MAX_ARGS	32
 
-extern char **environ;
 static const char *argv_exec_path;
 static const char *argv0_path;