I think you need not worry about this warning.I just googled for pruning registers and I got this link: **broken link removed**
So the warning you got, may be because the compiler must have noticed that some registers are not actually needed for your logic to work, and it must have optimized the design.
Check your code and see whether these signals - "reset" and "sccbclk_inv" changes.
What I mean is , if reset is always zero in your code, then the compiler will optimize the code into
Code:
if (rising_edge(sccbclk_inv)) then
sda_out <= sda;
end if;