lib/strings: remove redefinition `FlatString#[]`

Since `[]` is deprecated in strings (in favor of `.chars[]`), there
is no point of a fast implementation in FlatString.

Note that, `FlatStringCharView#[]` is already the fast path
(and is made faster by factorizing a field access)

Signed-off-by: Jean Privat <jean@pryen.org>