vinyl: use source tuple format when copying field map

There are two functions in vy_stmt.c that blindly copy tuple field map,
vy_stmt_dup() and vy_stmt_replace_from_upsert(). Both these functions
take a tuple format to use for the new statement and require this format
to be the same as the source tuple format in terms of fields definition,
otherwise they'll just crash. The only reason why we did that is that
back when these functions were written we used a separate format for
UPSERT statements so we needed this extra argument for creating a
REPLACE from UPSERT. Now it's not needed, and we can use the source
tuple format instead. Moreover, passing the current tuple format to any
of those functions is even harmful, because tuple format can be extended
by ALTER, in which case these functions will crash if called on a
statement created before ALTER. That being said, let's drop the tuple
format argument.