sql: remove unnecessary SCopy opcodes

Since OP_NoConflict opcode appears in INSERT/UPDATE VDBE listings
only when UNIQUE constraint check can't be handled by Tarantool
(after pushing 2255 to 2.0), related OP_SCopy should appear in
VDBE listing only when OP_NoConflict is present. This patch
contains a small fix for that.

Fix for #2255