sql: remove useless pragmas

The pragmas "query_only" and "read_uncommitted" didn't affect anything
and were removed.
Fixed an error in pragma index_list which caused a segmantation fault.
pragma sql_default_engine accepts only strings.
Thus pragma sql_default_engine('memtx') is a well-formed command,
while pragma sql_default_engine(memtx) or
pragma sql_default_engine("memtx") are considered as an ill-formed and
raise an error.

Closes #3733