cmd/compile: remove dead flags

cmd/compile: remove dead flags

For some time now, the -d flag has been used to control various named
debug options, rather than setting Debug['d']. Consequently, that
means dflag() always returns false, which means the -y flag is also
useless.

Similarly, Debug['L'] is never used anywhere, so the -L flag can be
dropped too.

Change-Id: I4bb12454e462410115ec4f5565facf76c5c2f255
Reviewed-on: https://go-review.googlesource.com/22121
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>