Remove FIXME asking why push_m is named push_m.

Since C doesn't support method overloading,
rb_ary_push() is for << and rb_ary_push_m() is for my_array.push.

The m most likely stands for 'method', as in push_method.

Signed-off-by: Vladimir Sizikov <vsizikov@gmail.com>

Author:    Rohit Arondekar <rohit.arondekar@gmail.com>