Custom outs*/ins* implementations are long gone from the xtensa port,
remove matching EXPORT_SYMBOLs.
This fixes the following build warnings issued by modpost since commit
15bfc234 ("modpost: check for static EXPORT_SYMBOL* functions"):

  WARNING: "insb" [vmlinux] is a static EXPORT_SYMBOL
  WARNING: "insw" [vmlinux] is a static EXPORT_SYMBOL
  WARNING: "insl" [vmlinux] is a static EXPORT_SYMBOL
  WARNING: "outsb" [vmlinux] is a static EXPORT_SYMBOL
  WARNING: "outsw" [vmlinux] is a static EXPORT_SYMBOL
  WARNING: "outsl" [vmlinux] is a static EXPORT_SYMBOL

Cc: stable@vger.kernel.org
Fixes: d38efc1f ("xtensa: adopt generic io routines")
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>