Remove unused abi attributes.

They've been replaced by putting the name on the extern block.

  #[abi = "foo"]

goes to

  extern "foo" { }

Closes #9483.
Removing ccdecl

as per https://github.com/mozilla/rust/pull/9606#discussion_r6930872
auto merge of #9606 : steveklabnik/rust/abi_removal, r=alexcrichton

They've been replaced by putting the name on the extern block.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