Disable translucency effect by default

Many reasons have been brought up over the years for why this effect is
not appropriate to be enabled by default:
- It was designed to highlight the whizzy technical feature of being
  able to make windows transparent, which is no longer particularly
  impressive today.
- It looks visually dated.
- It can produce a confusing visual soup by blending a window being
  moved or resized with the content below it, which we recently disabled
  for the Highlight Windows effect.
- If one window is covering up another as a way to deliberately hide the
  content of the lower window, this effect will reveal the hidden
  content whenever the upper window is moved or resized at all.

Overall it does not seem to have enough advantages to offset these
issues. Let's disable it by default--but only for the people who did not
modify its configuration at all. We can assume that those people like
it, so let's keep it on for them.

BUG: 384054
FIXED-IN: 5.23

Closes Phab task T7915