remove explicit declaration of putenv, ...

* src/date.c: ... now that gnulib guarantees that it's in <stdlib.h>.
* src/env.c: Likewise.

src/date.c

@@ -38,8 +38,6 @@
 
 #define AUTHORS proper_name ("David MacKenzie")
 
-int putenv ();
-
 static bool show_date (const char *format, struct timespec when);
 
 enum Time_spec

src/env.c

@@ -91,8 +91,6 @@
   proper_name ("Richard Mlynarik"), \
   proper_name ("David MacKenzie")
 
-int putenv ();
-
 extern char **environ;
 
 static struct option const longopts[] =