libaom: go back to building libraries however upstream decides

Presently this means static, instead of the shared we started
building after a recent change (my blame).

Darwin build breaks with this,
and it was introduced on a whim so instead of making this
platform-specific, go back to how things were previously.

pkgs/development/libraries/libaom/default.nix

@@ -14,10 +14,6 @@ stdenv.mkDerivation rec {
     yasm perl cmake pkgconfig python3
   ];
 
-  cmakeFlags = [
-    "-DBUILD_SHARED_LIBS=ON"
-  ];
-
   preConfigure = ''
     # build uses `git describe` to set the build version
     cat > $NIX_BUILD_TOP/git << "EOF"