Remove old univariant code from rust-lang.c

Remove old univariant code from rust-lang.c

Since moving Rust enum handling into dwarf2read.c, some old code for
handling univariant enums in rust-lang.c has been obsolete.  This
patch removes this code.

Tested on x86-64 Fedora 26, using rustc 1.23 (1.24 emits incorrect
DWARF for enums and so can't be used for this test).

2018-04-12  Tom Tromey  <tom@tromey.com>

	* rust-lang.c (rust_print_struct_def): Remove univariant code.
	(rust_evaluate_subexp): Likewise.