options: remove --respect-prop-shadow

A quick search on GitHub showing the _COMPTON_SHADOW is actively being
used (most noteably, by polybar). It is not a difficult thing to support,
and it doesn't have that much runtime overhead. So remove the option
that guards this feature, make it enabled by default.

(Side note, I saw lots of people putting this option in their config
files, but this is actually a command line only option.)

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/config.c

@@ -524,7 +524,6 @@ char *parse_config(options_t *opt, const char *config_file, bool *shadow_enable,
 	    .shadow_opacity = .75,
 	    .shadow_blacklist = NULL,
 	    .shadow_ignore_shaped = false,
-	    .respect_prop_shadow = false,
 	    .xinerama_shadow_crop = false,
 
 	    .fade_in_step = 0.028,