Removed -Ynotnull setting.

It never worked and it would periodically jump out and bite
someone. Slash and burn so new plants can take root. Eliminated
NotNullType and NotNullClass, internal elements. Removed notNull
method on Type, which no longer has a purpose. Retained 'def
isNotNull' because there are types which are known by construction
not to be null (ThisType, ConstantType as long as the constant
isn't null, etc.) and that's still useful to know.
Remove -Xcheck-null setting.

Maybe this was useful in some way; but no way I ever saw.
I have comments which tell me this is exposed in the IDE so
I left a stub.

I also removed mkCheckInit. That probably sounds like it's related
to -Xcheckinit. Guess again, guy-who-thinks-names-mean-things. It
was only used by -Xcheck-null.
SI-7247, deprecated NotNull.

Removed NotNull from tests and the parentage of AnyVal.

Removed the tests which were actually testing anything to
do with NotNull; massaged the others to forget NotNull and/or
not to name local things NotNull.
Completely remove isNotNull/notNull.

Restarted the Zamboni and collected these as well.
Author was convinced by reviewer that clinging to
isNotNull like Linus's security blanket will not
help us with landing a picture of the Great Pumpkin.
YAGNI, Charlie Brown.
Eliminated containsNull.

This was a little trickier than the previous. I introduced
a new method 'isBottomSubClass' which is the obvious complement
to the beloved 'isNonBottomSubClass'. In eliminating the two
call sites of containsNull I might have overshot the mark a
bit when I rewrote fourthTry and thirdTryRef, but who is going
to argue with such beauty as this.
Merge pull request #2244 from paulp/pr/notNull

SI-7247, deprecate NotNull.