sql: remove OP_LoadPtr

This opcode was used when Vdbe had to store key_def in P4 for
OP_OpenRead/Write, so P4 was occupied and OP_LoadPtr was used to
store space pointer in another opcode. But now P4 is free for
OpenRead/Write and space pointer can be stored here.

Alongside, some useless key_def duplications are removed.