Cleanup: delete unused IntSetToString

It was added in http://cr/19491949 (and was referenced at the time). The last reference was removed in http://cr/19507363.

This fixes warning/error with pre-release crosstoolv18:

'std::string leveldb::{anonymous}::IntSetToString(const std::set<long unsigned int>&)' defined but not used [-Werror=unused-function]

Added arm64 and and armv7s to IOS build as suggested on leveldb mailing list.

Changed local variable type from int to size_t

This eliminates compiler warning/error and resolves issue #146