Revert "linux-user: un-parent OBJECT(cpu) when closing thread"

This reverts commit 52f0c160.

This caused a regression in arm/aarch64.

We are hard-coding ARMCPRegInfo pointers into TranslationBlocks,
for calling into helper_{get,set}cp_reg{,64}.  So we have a race
condition between whichever cpu thread translates the code first
(encoding the pointer), and that cpu thread exiting, so that the
next execution of the TB references a freed data structure.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>