sql: remove expmask from prepared statement

expmask indicated necessity to recompile statement after parameter was
bound: it might turn out that parameter can affect query plan. However,
part of this mechanism has been removed long ago as a SQLite's legacy.
In its current state expmask is likely to be useless and assertions
involving it are obviously unsuitable. This patch completely removes
expmask and related routines.

Closes #4566