Removing block style for code-block about draft-js

[hellendag]

IIRC, it caused confusion in cases like this:

function helloWorld() {
^
}

Where the caret is at the start of the second line. In this case, when users backspaced, they didn't want to strip the block type -- they wanted to delete the line and move the cursor back to the previous line.

One way to resolve this would be to check whether the line above is code-block, and if not, make the current block unstyled.