opencv: cleanup depends and comments

Since the opencv package globally depends on BR2_TOOLCHAIN_HAS_THREADS
and BR2_USE_WCHAR, then cleanup these redundant (and useless)
dependencies in the opencv's sub-options.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>