Fix #231 backspacing non-symbols selecting

Intro'd by the following hunk (`this[dir].deleteTowards(dir, this)` on
commands that aren't symbols, like SquareRoot or Fraction, selects them;
`this.notify('edit')` thereafter immediately deletes the command):
https://github.com/mathquill/mathquill/commit/b548eaa4744f93d7acbca8c7b09ada149fd9bdcb#diff-400b59ae62decbf3a292ffc1ea08a73dL308
Cursor::deleteSelection needn't return anything #simplify

Since b548eaa4744f93d7acbca8c7b09ada149fd9bdcb, only called by an
onNotify() callback, where its return value is ignored.