accel: Remove unused AccelClass::opt_name attribute

accel: Remove unused AccelClass::opt_name attribute

The AccelType type was converted to AccelClass QOM
object on b14a0b7469f, and the original data type had
a field to store the option name which in turn was
used to search an accelerator. The lookup method
(accel_find) changed too, making the option field
unnecessary but it became AccelClass::opt_name despite
that. Therefore, and given that none accelerator
implementation sets AccelClass::opt_name, let's
remove this attribute.

Signed-off-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Message-Id: <20190531165334.20403-2-wainersm@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>