gdb: un-share set_inferior_cwd declaration

The declaration of set_inferior_cwd is currently shared between gdb and
gdbserver, in gdbsupport/common-inferior.h.  It doesn't need to be, as
set_inferior_cwd is not called from common code.  Only get_inferior_cwd
needs to.

The motivation for this is that a future patch will change the prototype
of set_inferior_cwd in gdb, and I don't want to change it for gdbserver
unnecessarily.  I see this as a good cleanup in any case, to reduce to
just the essential what is shared between GDB and GDBserver.

Change-Id: I3127d27d078f0503ebf5ccc6fddf14f212426a73

gdbsupport/common-inferior.h

@@ -36,10 +36,6 @@ extern const char *get_exec_file (int err);
    been set, then return NULL.  */
 extern const char *get_inferior_cwd ();
 
-/* Set the inferior current working directory.  If CWD is NULL, unset
-   the directory.  */
-extern void set_inferior_cwd (const char *cwd);
-
 /* Whether to start up the debuggee under a shell.
 
    If startup-with-shell is set, GDB's "run" will attempt to start up