remove the minor GC introduced in clear_all_old()
add prepare_major_gc()

clear_all_old() is pretty precise on what it does, but
prepare_major_gc() is more descriptive on what's its role
replace is_minor_gc() to is_generational() in mrb_incremental_gc()

in the current code, the only chance to trigger the major GC is
out of memory in mrb_realloc_simple(). it would be good to trigger
a major GC in mrb_incremental_gc() if nessecary, for the fact that
the only difference between major GC and minor GC is the difference
on white objects, we can keep the logic same.
fix the missing GC while switching off the generational mode