frv: Remove redundant debugging info flag

The generic makefile now sets '-Wa,--gdwarf2' in KBUILD_AFLAGS, so
this setting is no longer needed in arch makefiles.

Also remove a commented out addition of a -O1 to KBUILD_CFLAGS, and
comment text relating to these removed lines.

Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Geoff Levand <geoff@infradead.org> for Huawei, Linaro
Signed-off-by: Michal Marek <mmarek@suse.cz>