linux: Remove assembly umount2 implementation

Only alpha and ia64 do not support __NR_umount2 (defined as
__NR_umount), but recent kernel fixes (74cd2184833f for ia64, and
12b57c5c70f39 for alpha) add the required alias.

Checked with a build against all affected ABIs.

Reviewed-by: Florian Weimer <fweimer@redhat.com>

sysdeps/unix/sysv/linux/include/sys/mount.h

@@ -0,0 +1,31 @@
+/* Internal declarations for sys/mount.h.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#ifndef _INCLUDE_SYS_MOUNT_H
+#define _INCLUDE_SYS_MOUNT_H	1
+
+#include_next <sys/mount.h>
+
+# ifndef _ISOMAC
+
+extern __typeof (umount) __umount __THROW;
+extern __typeof (umount2) __umount2 __THROW;
+libc_hidden_proto (__umount2)
+
+# endif /* _ISOMAC */
+#endif /* sys/sysinfo.h */