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.
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>

doc/api/libinput.doxygen.in

@@ -10,7 +10,6 @@ MAX_INITIALIZER_LINES  = 0
 WARNINGS               = YES
 QUIET                  = YES
 INPUT                  = "@builddir@"
-FILTER_PATTERNS        = *.h *.dox
 IMAGE_PATH             = "@builddir@"
 GENERATE_HTML          = YES
 HTML_OUTPUT            = api