libobs,plugins: Remove new obs_output_*2 functions

Effectively reverting parts of d314d47, this commit removes the new
functions that got added to remove the flags parameter. Instead, it just
marks the parameter as unused and documents this. Having what is
effectively an API break just to remove a parameter is a bit overkill.
The other parts of d314d47 which cleaned up the usage of the flags
parameter are untouched here.
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