sh.lang: Avoid item recognition in "case … esac" statements

This MR follows !69 (closed), where I tried to improve item recognition in "case … esac" statements, without being able to reach a satisfactory result.

Since this recognition is not crucial (for comparison, it is not present in some well-known IDEs), I come to the conclusion that the best option is to remove it, in order to also remove side-effects which accompany it, concerning both syntax highlighting and performance.