Remove warning about unreachable bytecodes

[Per Bothner]

Tom Tromey wrote:

> Do we want a new option?
> Do we want if off by default?

My suggestion:

- Don't add a separate flag, at least until we see a use for it.
- Turn on the warning when extra_warnings is true (i.e. when -W
or -Wextra is specified).
- The default should be off by default.
- The libgcj Makefile should use -Wextra.
-- 
	--Per Bothner
per@bothner.com   http://per.bothner.com/