MIPS: Remove redundant addr_limit assignment on exec.

The address limit is already set in flush_old_exec() via set_fs(USER_DS)
so this assignment is redundant.

[ralf@linux-mips.org: also see dac853ae89043f1b7752875300faf614de43c74b for
further explanation.]

Signed-off-by: Mathias Krause <minipli@googlemail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/2466/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>