Drop obsolete layout.css.transition-behavior.enabled flag (#221)

There is no point in setting this flag to true when it's no longer doing
anything, `transition-behavior` is now enabled unconditionally.

Signed-off-by: Oriol Brufau <[email protected]>

Author: Loirooriol

stylo_static_prefs/src/lib.rs

@@ -33,9 +33,6 @@ macro_rules! pref {
     ("layout.css.stretch-size-keyword.enabled") => {
         true
     };
-    ("layout.css.transition-behavior.enabled") => {
-        true
-    };
     ("layout.css.marker.restricted") => {
         true
     };