build: Remove unnecessary DESTDIR from make_args

DESTDIR will not be used in both static and dynamic build because:
* In static build, nothing will be installed.
* In dynamic build, rbconfig.rb is explicitly told to mkmf and
  `RbConfig::CONFIG['prefix']` points to the directory prefixed by DESTDIR
  (usually derived from the location of rbconfig.rb itself).

lib/ruby_wasm/build/product/crossruby.rb

@@ -37,7 +37,6 @@ def make_args(crossruby)
       make_args << "AR=#{@toolchain.ar}"
       make_args << "RANLIB=#{@toolchain.ranlib}"
 
-      make_args << "DESTDIR=#{crossruby.dest_dir}"
       make_args
     end