Drop urxvt 1015 mouse extension
Let's see if we can drop this extension. Everyone should use SGR 1006 instead.
It's not a big win code-wise, but it would be a big win in the terminal emulation ecosystem to know that it's practically no longer used (or if it is, push for this goal).
Let's probably not drop it right now unconditionally, but we can make it conditional to a stable VTE version, or drop unconditionally at the beginning of the next devel cycle – to give more time to catch any regression.