btree: keys: remove unused method

I see no where used btree_key_t::fits; remove it.

Signed-off-by: Liu Aleaxander <Aleaxander@gmail.com>
unittest: fix wrong type error

Though that call is actually commented now, you will meet build error if
remove that comment.

Signed-off-by: Liu Aleaxander <Aleaxander@gmail.com>
unittest: var in a class should be private

"public" should be a typo here, and change it to "private" as we usally do.

Signed-off-by: Liu Aleaxander <Aleaxander@gmail.com>
Merge pull request #2271 from Aleaxander/yliu-btree-trivial

yet another tiny series of trivial fixes on btree