sql: remove nKey from struct BtCursor

nKey member (indicating number of parts in key) now is used only for
COUNT routine. On the other hand, it is always equal to 0 (as well as
key is really NULL), since functions which are relied on this argument
(tarantoolSqlite3Count and tarantoolSqlite3EphemeralCount) are called to
perform OP_Count opcode. In its turn, OP_Count is emitted only for
queries like <SELECT COUNT(*) FROM tab>, i.e. simple queries without
filter conditions. Hence, nothing prevents us from removing this field
at all.