Let's not present usage pop-ups for unsupported functions in custom expression interface

[lbrdnk]

Issue #34993 requires us to fix percentile support for mariadb, saying that user tried to use percentile in the custom expression interface but translation to sql which they've got was not right. However we do not support :percentile-expressions for mariadb on the backend. Let alone, mariadb supports only window version on percetile_disc and currently query builder also does not support use of window functions (related issue is #9393).

I believe the fact, that they were seeing the usage pop-up after writing in percentile( made them think it is something we are able to correctly compute from query builder and that's false.

1. Not showing that pop-up could make us less prone to get bug reports for bugs which are not bugs.
2. Same case for completion of arguments to functions that are not supported.

With regards to the completion of function name -- This is something we do not do for unsupported functions and that's right.

Attached screenshots contain (1) no completion when writing percentile and (2) pop-up after entring in percentile( and (3) completion of arguments that should imo happen only for supported functions.

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And super ideal behavior would be to not even enable user to generate request for query containing use of a function that is not supported by a driver or make backend return eg. 418 when request like this is processed.

[kamilmielnik]

I can still reproduce it in master, 3e4a40e.

Frontend does not know which functions are supported by which database, so this would need backend work as well.