doc/api: remove invalid doxygen option
doxygen 1.8.17 shows this error:
error: Illegal format for option FILTER_PATTERNS, no equal sign ('=') specified for item '*.h'
error: Illegal format for option FILTER_PATTERNS, no equal sign ('=') specified for item '*.dox'
This was added in deadbf35 but I cannot figure out how this ever had any effect based on the documentation for it. So let's drop it, I don't think it has any effect anyway.