Remove \I (ignore case) flag for find mode.
Remove \I (ignore case) flag for find mode.

It appears that nobody is using this (and it is broken).  So this PR
removes this feature.

Fixes #2937 (kind of).

decaffeinate-examples/vimium,smblott-github/vimium,poacher2k/vimium,poacher2k/vimium,smblott-github/vimium,philc/vimium,philc/vimium,gdh1995/vimium,smblott-github/vimium,poacher2k/vimium,gdh1995/vimium,gdh1995/vimium,decaffeinate-examples/vimium,decaffeinate-examples/vimium,philc/vimium