sql: drop unused functions

These functions are not used and should be removed. Most likely these
are not all unused functions, but they all use struct stmt, which will
be removed in the next commit.

Also, this patch removes the aCounter field from the struct Vdbe, since
the only function that uses this field is removed in this commit.

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring