libct/standard_init: fix linter warning
libct/standard_init: fix linter warning

The staticcheck linter points out that the err != nil comparison
after system.Exec is always true:

> libcontainer/standard_init_linux.go#L253
> SA4023: this comparison is always true (staticcheck)
> libcontainer/system/linux.go#L43
> SA4023(related information): github.com/opencontainers/runc/libcontainer/system.Exec never returns a nil interface value (staticcheck)

Indeed, Exec either returns an error or does not return at all.

Remove the (useless) check.

Signed-off-by: Kir Kolyshkin <3a017b8ddb3f9cf3e4a59978b004111bdeb97f08@gmail.com>

opencontainers/runc,kolyshkin/runc,kolyshkin/runc,kolyshkin/runc,opencontainers/runc,opencontainers/runc,opencontainers/runc,kolyshkin/runc