pythongh-63760: Solaris declares gethostname()

Since at least 2013, Oracle Solaris defines gethostname().
socketmodule.c no longer has to define gethostname() for Solaris.

Oracle Solaris and OpenSolaris have patches to remove the
gethostname() definition in Python:

* https://github.com/oracle/solaris-userland/blob/master/components/python/python37/patches/15-gethostname.patch
* https://github.com/OpenIndiana/oi-userland/blob/oi/hipster/components/python/python37/patches/15-gethostname.patch
* https://github.com/omniosorg/omnios-build/blob/master/build/python27/patches/24-gethostname.patch

Author: vstinner

Modules/socketmodule.c

@@ -111,11 +111,6 @@ Local naming conventions:
 #include "pycore_fileutils.h"     // _Py_set_inheritable()
 #include "pycore_moduleobject.h"  // _PyModule_GetState
 
-// gethostname() prototype missing from Solaris standard header files
-#ifdef __sun
-extern int gethostname(char *, int);
-#endif
-
 #ifdef _Py_MEMORY_SANITIZER
 #  include <sanitizer/msan_interface.h>
 #endif