Remove unused abi attributes.

They've been replaced by putting the name on the extern block.

  #[abi = "foo"]

goes to

  extern "foo" { }

Closes #9483.
auto merge of #9606 : steveklabnik/rust/abi_removal, r=huonw

They've been replaced by putting the name on the extern block.

  #[abi = "foo"]

goes to

  extern "foo" { }

Closes #9483.