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.h

@@ -149,8 +149,6 @@ typedef struct options {
 	c2_lptr_t *shadow_blacklist;
 	/// Whether bounding-shaped window should be ignored.
 	bool shadow_ignore_shaped;
-	/// Whether to respect _COMPTON_SHADOW.
-	bool respect_prop_shadow;
 	/// Whether to crop shadow to the very Xinerama screen.
 	bool xinerama_shadow_crop;