remote: relocate valid_remote_name

Move the `valid_remote_name()` function from the refspec subsystem to
the remote subsystem to better align with the separation of concerns.

Signed-off-by: Meet Soni <meetsoni3017@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

remote.c

@@ -3029,3 +3029,13 @@ char *relative_url(const char *remote_url, const char *url,
 	free(out);
 	return strbuf_detach(&sb, NULL);
 }
+
+int valid_remote_name(const char *name)
+{
+	int result;
+	struct strbuf refspec = STRBUF_INIT;
+	strbuf_addf(&refspec, "refs/heads/test:refs/remotes/%s/test", name);
+	result = valid_fetch_refspec(refspec.buf);
+	strbuf_release(&refspec);
+	return result;
+}