media-libs/libpng: enable RVV autodetecting

The code has been updated in the new version not to hardcode `-march`
setting anymore and will enable RVV extension based on the user-defined
settings

Bug: https://bugs.gentoo.org/958203
Signed-off-by: Jakov Smolić <jsmolic@gentoo.org>

media-libs/libpng/libpng-1.6.50-r1.ebuild

@@ -65,7 +65,6 @@ multilib_src_configure() {
 		$(use_enable test tests)
 		$(use_enable cpu_flags_x86_sse intel-sse)
 		$(use_enable static-libs static)
-		--disable-riscv-rvv
 	)
 
 	ECONF_SOURCE="${S}" econf "${myeconfargs[@]}"