gui-wm/river: drop unnecessary definition of src_unpack()

Since zig_src_unpack() is an exported phase function of zig.eclass, so
there is no need to explictily define src_unpack() (if no other
inherited eclass also exports src_unpack(), which is not the case for
this ebuild).

Signed-off-by: Florian Schmaus <flow@gentoo.org>

gui-wm/river/river-0.3.6.ebuild

@@ -52,10 +52,6 @@ PATCHES=( "${FILESDIR}/${P}-fix-no-lazypath.patch" )
 
 DOCS=( "README.md" )
 
-src_unpack() {
-	zig_src_unpack
-}
-
 src_configure() {
 	local my_zbs_args=(
 		-Dstrip=false # Let Portage control this