Remove unused single screenshot rule

This was never enabled in the EsLint config.
And considering we now use Unit test for this I am not sure this
is a requirement now.

Bug: none
Change-Id: I6fee042b123ea10722ca6d0cd86bfce3eeb01a62
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/6696129
Reviewed-by: Alex Rudenko <[email protected]>
Auto-Submit: Nikolay Vitkov <[email protected]>
Commit-Queue: Alex Rudenko <[email protected]>
Commit-Queue: Nikolay Vitkov <[email protected]>

Author: Lightning00Blade