Remove some variables but call functions for side effects

This patch consolidates the (possibly-questionable) spots where we
remove a declaration but continue to call some function for side
effects.  In a couple of cases it wasn't entirely clear to me that
this mattered; and in some other cases it might be more aesthetically
pleasing to use ATTRIBUTE_UNUSED.  So, I broke this out into a
separate patch for simpler review.

2016-07-14  Tom Tromey  <tom@tromey.com>

	* arch-utils.c (default_skip_permanent_breakpoint): Remove
	"bp_insn".
	* disasm.c (do_assembly_only): Remove "num_displayed".
	* dwarf2read.c (read_abbrev_offset): Remove "length".
	(dwarf_decode_macro_bytes) <DW_MACINFO_vendor_ext>: Remove
	"constant".
	* m32c-tdep.c (make_regs): Remove "r2hl", "r3hl", and "intbhl".
	* microblaze-tdep.c (microblaze_frame_cache): Remove "func".
	* tracefile.c (trace_save): Remove "status".