linux-user: Remove dead error-checking code

linux-user: Remove dead error-checking code

Remove some dead code spotted by Coverity (CID 1009855,
1390854, 1390847). The underlying cause in all these cases
is the same: QEMU's put_user operations can't result in
errors, but the kernel's equivalent does. So when code
was copied from the kernel signal-frame-setup/teardown
code, checks on error flags that were needed in the kernel
became dead code for us.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20181019161715.12122-1-peter.maydell@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>