net.box: fix crash when remote space field has unknown type

This commit fixes the following assertion failure that happens on
a client in case a remote schema contains an unknown field type:

  src/box/lua/misc.cc:395: int lbox_tuple_format_new(lua_State*):
  Assertion `fields[i].type != field_type_MAX' failed.

To fix the bug we remove the code that tries to set field types from
box.internal.new_tuple_format. Actually, the format is used solely for
providing field names so types are ignored anyway.

Closes #4632

NO_DOC=bug fix