search: remove PathPatternsAreCaseSensitive

Created by: rvantonder

Looks like PathPatternsAreCaseSensitive is always false, I audited every use not in test files: https://sourcegraph.com/search?q=repo:%5Egithub%5C.com/sourcegraph/sourcegraph%24+PathPatternsAreCaseSensitive+-file:test&patternType=literal. I think this is state that intended to capture whether file: values are case sensitive, but there is no associated logic. Best to remove until we have a need for this.

EDIT: It looks like this option is exclusively available in searcher but never used. This PR doesn't go ahead and remove the option from searcher protocol or its use in pathmatch.go, where local searcher tests actually depend on this behavior/interface. So, on second thought, feel free to oppose this PR since there seems to be some interface in place for this unused functionality. Can you comment your thoughts on this @keegancsmith?

The part that bothers me is that we don't use it anywhere, but define it in our types in frontend. I'm in the process of factoring out the inimitable getPatternInfo and noticed there isn't logic to carry over for PathPatternsAreCaseSensitive. The more stuff I can remove the easier this'll be. Current state of this PR removes PathPatternsAreCaseSensitive from the frontend (and never communicates it to searcher) but retains the definitions of PathPatternsAreCaseSensitive in the searcher protocol.