gdb: revert one array_view copy change in ada-lang.c

gdb: revert one array_view copy change in ada-lang.c

Commit 4bce7cdaf481 ("gdbsupport: add array_view copy function") caused
an internal error when running gdb.ada/packed_array_assign.exp:

    print pra(1) := pr^M
    /home/smarchi/src/binutils-gdb/gdb/../gdbsupport/array-view.h:217: internal-error: copy: Assertion `dest.size () == src.size ()' failed.^M

I am not sure what's the root cause of this, whether it is a GDB bug
exposed by using the array_view copy function or not.  Back out the
change that triggers the internal error for now, while we investigate
it.

Change-Id: I055ab14143e4cfd3ca7ce8f4855c6c3c05db52a7